diff --git a/pkg/rain/compile_bench_test.go b/pkg/rain/compile_bench_test.go
index 79c037b..371f326 100644
--- a/pkg/rain/compile_bench_test.go
+++ b/pkg/rain/compile_bench_test.go
@@ -76,6 +76,22 @@ func BenchmarkSelectToSQL(b *testing.B) {
 				ToSQL()
 		}
 	})
+
+	b.Run("InClause1000", func(b *testing.B) {
+		vals := make([]int64, 1000)
+		for i := range 1000 {
+			vals[i] = int64(i)
+		}
+
+		b.ReportAllocs()
+		b.ResetTimer()
+		for range b.N {
+			_, _, _ = db.Select().
+				Table(users).
+				Where(users.ID.In(vals...)).
+				ToSQL()
+		}
+	})
 }
 
 func BenchmarkInsertToSQL(b *testing.B) {
diff --git a/pkg/rain/query_compile.go b/pkg/rain/query_compile.go
index 83e5117..d4224d1 100644
--- a/pkg/rain/query_compile.go
+++ b/pkg/rain/query_compile.go
@@ -308,6 +308,8 @@ func (c *compileContext) writeJoinedPredicates(predicates []schema.Predicate, wr
 		return c.writePredicate(predicates[0])
 	}
 
+	c.ensureArgsCapacity(len(predicates))
+
 	if wrap {
 		c.writeByte('(')
 	}
@@ -398,6 +400,7 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 		default:
 			return fmt.Errorf("rain: invalid binary operator %q", value.Operator)
 		}
+		c.ensureArgsCapacity(2)
 		c.writeByte('(')
 		if err := c.writeExpression(value.Left); err != nil {
 			return err
@@ -413,6 +416,7 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 		if len(value.Exprs) < 2 {
 			return errors.New("rain: CONCAT requires at least two expressions")
 		}
+		c.ensureArgsCapacity(len(value.Exprs))
 		switch c.dialect.Name() {
 		case "postgres", "sqlite":
 			c.writeByte('(')
@@ -443,6 +447,7 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 		if len(value.Values) == 0 {
 			return errors.New("rain: IN predicate requires at least one value")
 		}
+		c.ensureArgsCapacity(len(value.Values) + 1)
 		if err := c.writeExpression(value.Left); err != nil {
 			return err
 		}
@@ -451,20 +456,23 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 		} else {
 			c.writeString(" IN (")
 		}
+
+		// OPTIMIZATION: Hoist expressionContext outside the loop and reuse it.
+		ctx := expressionContext{}
+		if len(value.Values) == 1 {
+			ctx.noParens = true
+		}
 		for idx, item := range value.Values {
 			if idx > 0 {
 				c.writeString(", ")
 			}
-			ctx := expressionContext{}
-			if len(value.Values) == 1 {
-				ctx.noParens = true
-			}
 			if err := c.writeExpressionInContext(item, ctx); err != nil {
 				return err
 			}
 		}
 		c.writeByte(')')
 	case schema.BetweenExpr:
+		c.ensureArgsCapacity(3)
 		if err := c.writeExpression(value.Left); err != nil {
 			return err
 		}
@@ -515,6 +523,7 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 			c.writeString(" IS NULL")
 		}
 	case schema.LogicalExpr:
+		c.ensureArgsCapacity(len(value.Exprs))
 		c.writeByte('(')
 		for idx, part := range value.Exprs {
 			if idx > 0 {
@@ -531,6 +540,16 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 		if len(value.WhenThenPairs) == 0 {
 			return errors.New("rain: CASE expression requires at least one WHEN clause")
 		}
+		// OPTIMIZATION: Hint capacity for WHEN/THEN pairs plus optional value and else.
+		n := len(value.WhenThenPairs) * 2
+		if value.ValueExpression != nil {
+			n++
+		}
+		if value.ElseExpression != nil {
+			n++
+		}
+		c.ensureArgsCapacity(n)
+
 		c.writeByte('(')
 		c.writeString("CASE")
 		if value.ValueExpression != nil {
@@ -584,6 +603,7 @@ func (c *compileContext) writeExpressionInContext(expr schema.Expression, contex
 		if len(value.Exprs) < 2 {
 			return errors.New("rain: COALESCE requires at least two expressions")
 		}
+		c.ensureArgsCapacity(len(value.Exprs))
 		c.writeString("COALESCE(")
 		for idx, part := range value.Exprs {
 			if part == nil {
@@ -631,6 +651,9 @@ func (c *compileContext) writeColumnName(column schema.ColumnReference) {
 }
 
 func (c *compileContext) writeRaw(raw schema.RawExpr) error {
+	if len(raw.Args) > 0 {
+		c.ensureArgsCapacity(len(raw.Args))
+	}
 	argIndex := 0
 	for idx := range len(raw.SQL) {
 		if raw.SQL[idx] != '?' {