diff --git a/build.mill b/build.mill
index bb6e8278..e6ee8a6b 100644
--- a/build.mill
+++ b/build.mill
@@ -53,9 +53,11 @@ trait SjsonnetCrossModule extends CrossScalaModule with ScalafmtModule {
     mvn"org.scala-lang.modules::scala-collection-compat::2.14.0"
   )
 
-  def manifest = super.manifest().add(
-    "Enable-Native-Access" -> "net.jpountz.util.Native"
-  )
+  def manifest = super
+    .manifest()
+    .add(
+      "Enable-Native-Access" -> "net.jpountz.util.Native"
+    )
 
   def generatedSources = Task {
     os.write(
diff --git a/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala b/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala
index 00b2af74..11fd2002 100644
--- a/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala
+++ b/sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala
@@ -213,7 +213,9 @@ object ArrayModule extends AbstractFunctionModule {
   private object FlattenArrays extends Val.Builtin1("flattenArrays", "arrs") {
     def evalRhs(arrs: Lazy, ev: EvalScope, pos: Position): Val = {
       val out = new mutable.ArrayBuilder.ofRef[Lazy]
-      for (x <- arrs.force.asArr) {
+      val arr = arrs.force.asArr
+      out.sizeHint(arr.length * 4) // Rough size hint
+      for (x <- arr) {
         x.force match {
           case Val.Null(_) => // do nothing
           case v: Val.Arr  => out ++= v.asLazyArray
@@ -226,9 +228,11 @@ object ArrayModule extends AbstractFunctionModule {
 
   private object FlattenDeepArrays extends Val.Builtin1("flattenDeepArray", "value") {
     def evalRhs(value: Lazy, ev: EvalScope, pos: Position): Val = {
+      val lazyArray = value.force.asArr.asLazyArray
       val out = new mutable.ArrayBuilder.ofRef[Lazy]
-      val q = new java.util.ArrayDeque[Lazy]()
-      value.force.asArr.asLazyArray.foreach(q.add)
+      out.sizeHint(lazyArray.length)
+      val q = new java.util.ArrayDeque[Lazy](lazyArray.length)
+      lazyArray.foreach(q.add)
       while (!q.isEmpty) {
         q.removeFirst().force match {
           case v: Val.Arr => v.asLazyArray.reverseIterator.foreach(q.push)
diff --git a/sjsonnet/src/sjsonnet/stdlib/StringModule.scala b/sjsonnet/src/sjsonnet/stdlib/StringModule.scala
index 7f44b316..d7c01dfb 100644
--- a/sjsonnet/src/sjsonnet/stdlib/StringModule.scala
+++ b/sjsonnet/src/sjsonnet/stdlib/StringModule.scala
@@ -234,6 +234,7 @@ object StringModule extends AbstractFunctionModule {
     }
 
     val b = new mutable.ArrayBuilder.ofRef[Lazy]
+    b.sizeHint(maxSplits)
     var sz = 0
     var i = 0
     var start = 0
@@ -313,8 +314,16 @@ object StringModule extends AbstractFunctionModule {
   }
 
   private object EncodeUTF8 extends Val.Builtin1("encodeUTF8", "str") {
-    def evalRhs(s: Lazy, ev: EvalScope, pos: Position): Val =
-      Val.Arr(pos, s.force.asString.getBytes(UTF_8).map(i => Val.Num(pos, i & 0xff)))
+    def evalRhs(s: Lazy, ev: EvalScope, pos: Position): Val = {
+      val bytes = s.force.asString.getBytes(UTF_8)
+      val arr = new Array[Lazy](bytes.length)
+      var i = 0
+      while (i < bytes.length) {
+        arr(i) = Val.Num(pos, bytes(i) & 0xff)
+        i += 1
+      }
+      Val.Arr(pos, arr)
+    }
   }
 
   private object DecodeUTF8 extends Val.Builtin1("decodeUTF8", "arr") {