diff --git a/classlib/src/main/java/org/teavm/classlib/java/lang/SystemNativeGenerator.java b/classlib/src/main/java/org/teavm/classlib/java/lang/SystemNativeGenerator.java
index f8d6ea108..2837c81b0 100644
--- a/classlib/src/main/java/org/teavm/classlib/java/lang/SystemNativeGenerator.java
+++ b/classlib/src/main/java/org/teavm/classlib/java/lang/SystemNativeGenerator.java
@@ -53,6 +53,16 @@ public class SystemNativeGenerator implements Generator, DependencyPlugin {
         String dest = context.getParameterName(3);
         String destPos = context.getParameterName(4);
         String length = context.getParameterName(5);
+        writer.append("if").ws().append("(").append(length).ws().append("===").ws().append("0)").ws().append("{")
+                .indent().softNewLine();
+        writer.append("return;").ws().softNewLine();
+        writer.outdent().append("}").ws().append("else ");
+        writer.append("if").ws().append("(typeof " + src + ".data.buffer").ws().append("!==").ws()
+                .append("'undefined')").ws().append("{").indent().softNewLine();
+        writer.append(dest + ".data.set(" + src + ".data.subarray(" + srcPos + ",").ws()
+                .append(srcPos).ws().append("+").ws().append(length).append("),").ws()
+                .append(destPos).append(");").softNewLine();
+        writer.outdent().append("}").ws().append("else ");
         writer.append("if (" + src + " !== " +  dest + " || " + destPos + " < " + srcPos + ") {").indent().newLine();
         writer.append("for (var i = 0; i < " + length + "; i = (i + 1) | 0) {").indent().softNewLine();
         writer.append(dest + ".data[" + destPos + "++] = " + src + ".data[" + srcPos + "++];").softNewLine();
diff --git a/classlib/src/main/java/org/teavm/classlib/java/lang/TString.java b/classlib/src/main/java/org/teavm/classlib/java/lang/TString.java
index baf3f02d0..272145bf2 100644
--- a/classlib/src/main/java/org/teavm/classlib/java/lang/TString.java
+++ b/classlib/src/main/java/org/teavm/classlib/java/lang/TString.java
@@ -47,17 +47,12 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
     }
 
     public TString(char[] characters) {
-        this.characters = new char[characters.length];
-        for (int i = 0; i < characters.length; ++i) {
-            this.characters[i] = characters[i];
-        }
+        this(characters, 0, characters.length);
     }
 
     public TString(char[] value, int offset, int count) {
         this.characters = new char[count];
-        for (int i = 0; i < count; ++i) {
-            this.characters[i] = value[i + offset];
-        }
+        System.arraycopy(value, offset, this.characters, 0, count);
     }
 
     static TString fromArray(char[] characters) {
@@ -177,9 +172,7 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
                 || dstBegin + (srcEnd - srcBegin) > dst.length) {
             throw new TIndexOutOfBoundsException();
         }
-        while (srcBegin < srcEnd) {
-            dst[dstBegin++] = charAt(srcBegin++);
-        }
+        System.arraycopy(characters, srcBegin, dst, dstBegin, srcEnd - srcBegin);
     }
 
     public boolean contentEquals(TStringBuffer buffer) {