diff --git a/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java b/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java
index 2100b14233..a0d186e74b 100644
--- a/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java
+++ b/common/src/main/java/io/netty/util/internal/InternalThreadLocalMap.java
@@ -18,6 +18,8 @@ package io.netty.util.internal;
 
 import io.netty.util.concurrent.FastThreadLocal;
 import io.netty.util.concurrent.FastThreadLocalThread;
+import io.netty.util.internal.logging.InternalLogger;
+import io.netty.util.internal.logging.InternalLoggerFactory;
 
 import java.nio.charset.Charset;
 import java.nio.charset.CharsetDecoder;
@@ -35,10 +37,23 @@ import java.util.WeakHashMap;
  */
 public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap {
 
+    private static final InternalLogger logger = InternalLoggerFactory.getInstance(InternalThreadLocalMap.class);
+
     private static final int DEFAULT_ARRAY_LIST_INITIAL_CAPACITY = 8;
+    private static final int STRING_BUILDER_INITIAL_SIZE;
+    private static final int STRING_BUILDER_MAX_SIZE;
 
     public static final Object UNSET = new Object();
 
+    static {
+        STRING_BUILDER_INITIAL_SIZE =
+                SystemPropertyUtil.getInt("io.netty.threadLocalMap.stringBuilder.initialSize", 1024);
+        logger.debug("-Dio.netty.threadLocalMap.stringBuilder.initialSize: {}", STRING_BUILDER_INITIAL_SIZE);
+
+        STRING_BUILDER_MAX_SIZE = SystemPropertyUtil.getInt("io.netty.threadLocalMap.stringBuilder.maxSize", 1024 * 4);
+        logger.debug("-Dio.netty.threadLocalMap.stringBuilder.maxSize: {}", STRING_BUILDER_MAX_SIZE);
+    }
+
     public static InternalThreadLocalMap getIfSet() {
         Thread thread = Thread.currentThread();
         if (thread instanceof FastThreadLocalThread) {
@@ -163,17 +178,16 @@ public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap
     }
 
     public StringBuilder stringBuilder() {
-        final int stringBuilderCapacity = 1024;
-        if (stringBuilder == null) {
-            stringBuilder = new StringBuilder(stringBuilderCapacity);
-        } else {
-            if (stringBuilder.capacity() > stringBuilderCapacity) {
-                stringBuilder.setLength(stringBuilderCapacity);
-                stringBuilder.trimToSize();
-            }
-            stringBuilder.setLength(0);
+        StringBuilder sb = stringBuilder;
+        if (sb == null) {
+            return stringBuilder = new StringBuilder(STRING_BUILDER_INITIAL_SIZE);
         }
-        return stringBuilder;
+        if (sb.capacity() > STRING_BUILDER_MAX_SIZE) {
+            sb.setLength(STRING_BUILDER_INITIAL_SIZE);
+            sb.trimToSize();
+        }
+        sb.setLength(0);
+        return sb;
     }
 
     public Map<Charset, CharsetEncoder> charsetEncoderCache() {