diff --git a/datetime.go b/datetime.go
index 481fd81..54b6e4f 100644
--- a/datetime.go
+++ b/datetime.go
@@ -5,8 +5,6 @@ package interpres
 
 import (
 	"fmt"
-	"regexp"
-	"strconv"
 	"strings"
 	"time"
 )
@@ -44,7 +42,8 @@ func (odt OffsetDateTime) String() string { return offsetString(odt.Time) }
 // fractional second. TOML 1.1 makes the seconds optional, so they appear only
 // when they are non-zero, and a fraction drops its trailing zeros.
 func (ldt LocalDateTime) String() string {
-	return ldt.Format("2006-01-02T") + clockString(ldt.Time)
+	buf := ldt.Time.AppendFormat(make([]byte, 0, 32), "2006-01-02T")
+	return string(appendClock(buf, ldt.Time))
 }
 
 // String returns the TOML-canonical rendering of the local date, e.g.
@@ -55,70 +54,187 @@ func (ld LocalDate) String() string { return ld.Format("2006-01-02") }
 // or "07:32:00.5" when the time carries a fractional second.
 func (lt LocalTime) String() string { return clockString(lt.Time) }
 
-// clockString renders a time of day the way TOML writes it: the seconds appear
-// only when the value carries them, and a fractional second drops its trailing
-// zeros, so half a second is "00.5" and not "00.500000000". Both are the same
-// value either way; the shorter form is the one TOML 1.1 allows.
+// appendClock appends the clock part of a TOML time to buf: HH:MM, seconds
+// only when the value carries them, and a fraction with its trailing zeros
+// dropped, so half a second is ".5" and not ".500000000". Both are the same
+// value either way; the shorter form is the one TOML 1.1 allows. The whole
+// rendering is built in one buffer, because the encoder writes a date-time
+// per entry of a large document.
+func appendClock(buf []byte, t time.Time) []byte {
+	buf = t.AppendFormat(buf, "15:04")
+	if t.Second() != 0 || t.Nanosecond() != 0 {
+		buf = t.AppendFormat(buf, ":05")
+	}
+	if ns := t.Nanosecond(); ns > 0 {
+		buf = append(buf, '.')
+		buf = append(buf, strings.TrimRight(fmt.Sprintf("%09d", ns), "0")...)
+	}
+	return buf
+}
+
+// clockString renders a time of day the way TOML writes it.
 func clockString(t time.Time) string {
-	out := t.Format("15:04")
-	ns := t.Nanosecond()
-	if t.Second() != 0 || ns != 0 {
-		out += t.Format(":05")
-	}
-	if ns > 0 {
-		out += "." + strings.TrimRight(fmt.Sprintf("%09d", ns), "0")
-	}
-	return out
+	return string(appendClock(make([]byte, 0, 16), t))
 }
 
 // offsetString renders an offset date-time, the fourth TOML kind, in the same
 // shape: no zero seconds, no trailing zeros in the fraction, and the offset
 // written as "Z" when it is zero.
 func offsetString(t time.Time) string {
-	return t.Format("2006-01-02T") + clockString(t) + t.Format("Z07:00")
+	buf := t.AppendFormat(make([]byte, 0, 32), "2006-01-02T")
+	buf = appendClock(buf, t)
+	buf = t.AppendFormat(buf, "Z07:00")
+	return string(buf)
 }
 
-var (
-	offsetDateTimeLayouts = []string{
-		"2006-01-02T15:04:05.999999999Z07:00",
-		"2006-01-02T15:04:05Z07:00",
-		"2006-01-02 15:04:05.999999999Z07:00",
-		"2006-01-02 15:04:05Z07:00",
-		// TOML 1.1 makes the seconds optional.
-		"2006-01-02T15:04Z07:00",
-		"2006-01-02 15:04Z07:00",
-	}
-	localDateTimeLayouts = []string{
-		"2006-01-02T15:04:05.999999999",
-		"2006-01-02T15:04:05",
-		"2006-01-02 15:04:05.999999999",
-		"2006-01-02 15:04:05",
-		"2006-01-02T15:04",
-		"2006-01-02 15:04",
-	}
-	localTimeLayouts = []string{
-		"15:04:05.999999999",
-		"15:04:05",
-		"15:04",
-	}
+// dateTimeKind names the date-time shape a bare token has, as the scanner
+// below classifies it.
+type dateTimeKind int
+
+const (
+	dateTimeNone dateTimeKind = iota
+	dateTimeOffset
+	dateTimeLocal
+	dateTimeDate
+	dateTimeClock
 )
 
-// dateTimeShape enforces the strict TOML grammar (two-digit components,
-// seconds optional since 1.1, a fraction only after seconds) that time.Parse
-// would otherwise accept loosely (e.g. a single-digit hour).
-var dateTimeShape = regexp.MustCompile(
-	`^\d{4}-\d{2}-\d{2}([Tt ]\d{2}:\d{2}(:\d{2}(\.\d+)?)?([Zz]|[+-]\d{2}:\d{2})?)?$` +
-		`|^\d{2}:\d{2}(:\d{2}(\.\d+)?)?$`,
+// The layouts the time package parses each shape with. Parsing accepts a
+// fractional second even when the layout does not carry one, so each shape
+// needs a single layout, chosen by whether the token has seconds.
+const (
+	offsetDateTimeLayout = "2006-01-02T15:04:05Z07:00"
+	offsetClockLayout    = "2006-01-02T15:04Z07:00"
+	localDateTimeLayout  = "2006-01-02T15:04:05"
+	localClockLayout     = "2006-01-02T15:04"
+	localTimeLayout      = "15:04:05"
+	localTimeClockLayout = "15:04"
+	localDateOnlyLayout  = "2006-01-02"
 )
 
-// offsetBounds extracts the numeric offset of a date-time. The ABNF bounds it
-// to 00:00 through 23:59, but time.Parse accepts values outside that range
-// and rolls them over (for example "+00:60" becomes "+01:00"), so the bounds
-// are enforced here.
-var offsetBounds = regexp.MustCompile(`([+-])(\d{2}):(\d{2})$`)
+// scanDateTimeShape validates a bare token against the strict TOML date-time
+// grammar and reports which kind it is: two-digit components, seconds
+// optional since TOML 1.1, a fraction only after seconds, an offset only
+// after a time, and an offset bounded to 00:00 through 23:59. The grammar is
+// a fixed byte shape, so the scan is a byte walk; the regular expressions
+// this replaced cost the parser measurably per token, and a shape that fails
+// the scan is simply not a date-time.
+func scanDateTimeShape(tok string) (kind dateTimeKind, seconds bool) {
+	// A local clock on its own: HH:MM[:SS[.fraction]].
+	if len(tok) >= 5 && tok[2] == ':' {
+		n, secs, ok := scanClock(tok, 0)
+		if !ok || n != len(tok) {
+			return dateTimeNone, false
+		}
+		return dateTimeClock, secs
+	}
+	// A date, optionally followed by a time and an offset.
+	if len(tok) < 10 || tok[4] != '-' || tok[7] != '-' {
+		return dateTimeNone, false
+	}
+	for _, i := range [8]int{0, 1, 2, 3, 5, 6, 8, 9} {
+		if !isDecDigit(tok[i]) {
+			return dateTimeNone, false
+		}
+	}
+	if len(tok) == 10 {
+		return dateTimeDate, false
+	}
+	if sep := tok[10]; sep != 'T' && sep != 't' && sep != ' ' {
+		return dateTimeNone, false
+	}
+	n, secs, ok := scanClock(tok, 11)
+	if !ok {
+		return dateTimeNone, false
+	}
+	if n == len(tok) {
+		return dateTimeLocal, secs
+	}
+	// The offset: Z/z, or a signed HH:MM bounded as the ABNF requires.
+	switch c := tok[n]; {
+	case c == 'Z' || c == 'z':
+		if n+1 != len(tok) {
+			return dateTimeNone, false
+		}
+	case c == '+' || c == '-':
+		if n+6 != len(tok) || tok[n+3] != ':' ||
+			!isDecDigit(tok[n+1]) || !isDecDigit(tok[n+2]) ||
+			!isDecDigit(tok[n+4]) || !isDecDigit(tok[n+5]) ||
+			tok[n+1] > '2' || (tok[n+1] == '2' && tok[n+2] > '3') ||
+			tok[n+4] > '5' {
+			return dateTimeNone, false
+		}
+	default:
+		return dateTimeNone, false
+	}
+	return dateTimeOffset, secs
+}
+
+// scanClock validates HH:MM[:SS[.fraction]] starting at i and returns the
+// position after the clock, whether seconds were present, and whether the
+// shape is valid.
+func scanClock(tok string, i int) (pos int, seconds bool, ok bool) {
+	if i+5 > len(tok) || tok[i+2] != ':' ||
+		!isDecDigit(tok[i]) || !isDecDigit(tok[i+1]) ||
+		!isDecDigit(tok[i+3]) || !isDecDigit(tok[i+4]) {
+		return 0, false, false
+	}
+	i += 5
+	if i == len(tok) || tok[i] != ':' {
+		return i, false, true
+	}
+	if i+3 > len(tok) || !isDecDigit(tok[i+1]) || !isDecDigit(tok[i+2]) {
+		return 0, false, false
+	}
+	i += 3
+	if i == len(tok) || tok[i] != '.' {
+		return i, true, true
+	}
+	i++
+	digits := i
+	for i < len(tok) && isDecDigit(tok[i]) {
+		i++
+	}
+	if i == digits {
+		return 0, false, false
+	}
+	return i, true, true
+}
+
+// normaliseDateTimeToken rewrites the date/time separator to 'T' and the
+// offset marker to 'Z', the characters the layouts above carry. A token that
+// already has them is returned as it is, without a copy.
+func normaliseDateTimeToken(tok string, kind dateTimeKind) string {
+	if kind == dateTimeDate || kind == dateTimeClock {
+		return tok
+	}
+	needs := false
+	for i := range len(tok) {
+		c := tok[i]
+		if c == 't' || c == 'z' || (c == ' ' && i == 10) {
+			needs = true
+			break
+		}
+	}
+	if !needs {
+		return tok
+	}
+	b := []byte(tok)
+	for i, c := range b {
+		switch {
+		case c == 't':
+			b[i] = 'T'
+		case c == 'z':
+			b[i] = 'Z'
+		case c == ' ' && i == 10:
+			b[i] = 'T'
+		}
+	}
+	return string(b)
+}
 
 // parseDateTime classifies and parses a bare token as a TOML date-time value.
-// It returns the decoded value (time.Time, LocalDateTime, LocalDate, or
+// It returns the decoded value (OffsetDateTime, LocalDateTime, LocalDate or
 // LocalTime) and whether the token was a date-time at all.
 func parseDateTime(tok string) (any, bool) {
 	if tok == "" || tok[0] < '0' || tok[0] > '9' {
@@ -127,35 +243,48 @@ func parseDateTime(tok string) (any, bool) {
 	if !strings.ContainsAny(tok, "-:") {
 		return nil, false
 	}
-	if !dateTimeShape.MatchString(tok) {
+	kind, seconds := scanDateTimeShape(tok)
+	if kind == dateTimeNone {
 		return nil, false
 	}
-	if m := offsetBounds.FindStringSubmatch(tok); m != nil {
-		hour, _ := strconv.Atoi(m[2])
-		minute, _ := strconv.Atoi(m[3])
-		if hour > 23 || minute > 59 {
+	norm := normaliseDateTimeToken(tok, kind)
+	switch kind {
+	case dateTimeOffset:
+		layout := offsetClockLayout
+		if seconds {
+			layout = offsetDateTimeLayout
+		}
+		t, err := time.Parse(layout, norm)
+		if err != nil {
 			return nil, false
 		}
-	}
-	// The ABNF accepts lowercase "t"/"z"; time.Parse only matches uppercase.
-	norm := strings.ToUpper(tok)
-	for _, layout := range offsetDateTimeLayouts {
-		if t, err := time.Parse(layout, norm); err == nil {
-			return OffsetDateTime{t}, true
+		return OffsetDateTime{t}, true
+	case dateTimeLocal:
+		layout := localClockLayout
+		if seconds {
+			layout = localDateTimeLayout
 		}
-	}
-	for _, layout := range localDateTimeLayouts {
-		if t, err := time.Parse(layout, norm); err == nil {
-			return LocalDateTime{t}, true
+		t, err := time.Parse(layout, norm)
+		if err != nil {
+			return nil, false
+		}
+		return LocalDateTime{t}, true
+	case dateTimeDate:
+		t, err := time.Parse(localDateOnlyLayout, norm)
+		if err != nil {
+			return nil, false
 		}
-	}
-	if t, err := time.Parse("2006-01-02", norm); err == nil {
 		return LocalDate{t}, true
-	}
-	for _, layout := range localTimeLayouts {
-		if t, err := time.Parse(layout, norm); err == nil {
-			return LocalTime{t}, true
+	case dateTimeClock:
+		layout := localTimeClockLayout
+		if seconds {
+			layout = localTimeLayout
 		}
+		t, err := time.Parse(layout, norm)
+		if err != nil {
+			return nil, false
+		}
+		return LocalTime{t}, true
 	}
 	return nil, false
 }