diff --git a/benchmarks/src/jmh/kotlin/PythonDateTimeFormatBenchmark.kt b/benchmarks/src/jmh/kotlin/PythonDateTimeFormatBenchmark.kt
new file mode 100644
index 000000000..24e3bbfcb
--- /dev/null
+++ b/benchmarks/src/jmh/kotlin/PythonDateTimeFormatBenchmark.kt
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+@file:Suppress("unused")
+
+package kotlinx.datetime
+
+import kotlinx.datetime.format.char
+import kotlinx.datetime.format.optional
+import org.openjdk.jmh.annotations.*
+import org.openjdk.jmh.infra.Blackhole
+import java.util.concurrent.*
+
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+@Fork(1)
+open class PythonDateTimeFormatBenchmark {
+
+    @Benchmark
+    fun buildPythonDateTimeFormat(blackhole: Blackhole) {
+        val v = LocalDateTime.Format {
+            year()
+            char('-')
+            monthNumber()
+            char('-')
+            day()
+            char(' ')
+            hour()
+            char(':')
+            minute()
+            optional {
+                char(':')
+                second()
+                optional {
+                    char('.')
+                    secondFraction()
+                }
+            }
+        }
+        blackhole.consume(v)
+    }
+}
diff --git a/benchmarks/src/jmh/kotlin/SerialFormatBenchmark.kt b/benchmarks/src/jmh/kotlin/SerialFormatBenchmark.kt
new file mode 100644
index 000000000..fb63f577e
--- /dev/null
+++ b/benchmarks/src/jmh/kotlin/SerialFormatBenchmark.kt
@@ -0,0 +1,45 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+@file:Suppress("unused")
+
+package kotlinx.datetime
+
+import kotlinx.datetime.format.char
+import org.openjdk.jmh.annotations.*
+import org.openjdk.jmh.infra.Blackhole
+import java.util.concurrent.TimeUnit
+
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Benchmark)
+@Fork(1)
+open class SerialFormatBenchmark {
+
+    @Param("1", "2", "4", "8", "16", "32", "64", "128", "256", "512", "1024")
+    var n = 0
+
+    @Benchmark
+    fun largeSerialFormat(blackhole: Blackhole) {
+        val format = LocalDateTime.Format {
+            repeat(n) {
+                char('^')
+                monthNumber()
+                char('&')
+                day()
+                char('!')
+                hour()
+                char('$')
+                minute()
+                char('#')
+                second()
+                char('@')
+            }
+        }
+        blackhole.consume(format)
+    }
+}
diff --git a/core/common/src/internal/format/parser/ConcatenatedListView.kt b/core/common/src/internal/format/parser/ConcatenatedListView.kt
new file mode 100644
index 000000000..3f11c3826
--- /dev/null
+++ b/core/common/src/internal/format/parser/ConcatenatedListView.kt
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2019-2025 JetBrains s.r.o. and contributors.
+ * Use of this source code is governed by the Apache 2.0 License that can be found in the LICENSE.txt file.
+ */
+
+package kotlinx.datetime.internal.format.parser
+
+internal class ConcatenatedListView<T>(val list1: List<T>, val list2: List<T>) : AbstractList<T>() {
+    override val size: Int
+        get() = list1.size + list2.size
+
+    override fun get(index: Int): T = if (index < list1.size) list1[index] else list2[index - list1.size]
+
+    override fun iterator(): Iterator<T> = ConcatenatedListViewIterator()
+
+    private inner class ConcatenatedListViewIterator : Iterator<T> {
+        private val iterators: List<Iterator<T>> = buildList {
+            collectIterators(list1)
+            collectIterators(list2)
+        }
+        private var index = 0
+
+        private fun MutableList<Iterator<T>>.collectIterators(list: List<T>) {
+            if (list is ConcatenatedListView<T>) {
+                collectIterators(list.list1)
+                collectIterators(list.list2)
+            } else {
+                add(list.iterator())
+            }
+        }
+
+        override fun hasNext(): Boolean {
+            while (index < iterators.size && !iterators[index].hasNext()) {
+                index++
+            }
+            return index < iterators.size
+        }
+
+        override fun next(): T = iterators[index].next()
+    }
+}
diff --git a/core/common/src/internal/format/parser/Parser.kt b/core/common/src/internal/format/parser/Parser.kt
index 9958e3fb9..27fc9d277 100644
--- a/core/common/src/internal/format/parser/Parser.kt
+++ b/core/common/src/internal/format/parser/Parser.kt
@@ -44,7 +44,7 @@ internal class ParserStructure<in Output>(
 // TODO: O(size of the resulting parser ^ 2), but can be O(size of the resulting parser)
 internal fun <T> List<ParserStructure<T>>.concat(): ParserStructure<T> {
     fun <T> ParserStructure<T>.append(other: ParserStructure<T>): ParserStructure<T> = if (followedBy.isEmpty()) {
-        ParserStructure(operations + other.operations, other.followedBy)
+        ParserStructure(ConcatenatedListView(operations, other.operations), other.followedBy)
     } else {
         ParserStructure(operations, followedBy.map { it.append(other) })
     }