diff --git a/src/main/cpp/blaze.cc b/src/main/cpp/blaze.cc
index 09760fc..a99931b 100644
--- a/src/main/cpp/blaze.cc
+++ b/src/main/cpp/blaze.cc
@@ -420,8 +420,12 @@
 
   // TODO(b/109998449): only assume JDK >= 9 for embedded JDKs
   if (!globals->options->GetEmbeddedJavabase().empty()) {
-    // quiet warnings from com.google.protobuf.UnsafeUtil,
+    // In JDK9 we have seen a slow down when using the default G1 collector
+    // and thus switch back to parallel gc.
+    result.push_back("-XX:+UseParallelOldGC");
     // see: https://github.com/google/protobuf/issues/3781
+
+    // quiet warnings from com.google.protobuf.UnsafeUtil,
     result.push_back("--add-opens=java.base/java.nio=ALL-UNNAMED");
   }
 
diff --git a/tools/jdk/default_java_toolchain.bzl b/tools/jdk/default_java_toolchain.bzl
index f4dc035..c518d026 100644
--- a/tools/jdk/default_java_toolchain.bzl
+++ b/tools/jdk/default_java_toolchain.bzl
@@ -19,6 +19,10 @@
 ]
 
 JDK9_JVM_OPTS = [
+    # In JDK9 we have seen a ~30% slow down in JavaBuilder performance when using
+    # G1 collector and having compact strings enabled.
+    "-XX:+UseParallelOldGC",
+    "-XX:-CompactStrings",
     # Allow JavaBuilder to access internal javac APIs.
     "--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
     "--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",