diff --git a/gc/mmtk/depend b/gc/mmtk/depend
index 1689f9d..79f9200 100644
--- a/gc/mmtk/depend
+++ b/gc/mmtk/depend
@@ -2,10 +2,22 @@ $(TARGET_SO): $(MMTK_BUILD)/$(LIBMMTK_RUBY)
 
 $(OBJS): Makefile
 
+# A raised LLVM inline threshold for the Rust GC code: this is what fuses the
+# per-object marking pipeline (work packet loop, closure trampoline, type fast
+# paths, mark enqueue) into straight-line code.  Without it the fast paths in
+# scanning.rs stay behind real calls and the win evaporates.
+#
+# The flag must reach rustc either via the target-scoped RUSTFLAGS in
+# MMTK_CARGO_ENV (LTO builds, see extconf.rb) or via plain RUSTFLAGS.  Never
+# set both: plain RUSTFLAGS has higher precedence in Cargo and would clobber
+# the target-scoped flags.
+MMTK_INLINE_THRESHOLD = 5000
+MMTK_THRESHOLD_RUSTFLAGS = -Cllvm-args=-inline-threshold=$(MMTK_INLINE_THRESHOLD)
+
 release/$(LIBMMTK_RUBY) debug/$(LIBMMTK_RUBY): $(RUSTSRCS) $(srcdir)/Cargo.toml $(srcdir)/Cargo.lock Makefile
 
 release/$(LIBMMTK_RUBY):
-	CARGO_TARGET_DIR="." $(MMTK_CARGO_ENV) cargo build --manifest-path=$(srcdir)/Cargo.toml --release $(MMTK_CARGO_FLAGS)
+	CARGO_TARGET_DIR="." $(if $(MMTK_CARGO_ENV),,RUSTFLAGS="$(RUSTFLAGS) $(MMTK_THRESHOLD_RUSTFLAGS)") $(MMTK_CARGO_ENV) cargo build --manifest-path=$(srcdir)/Cargo.toml --release $(MMTK_CARGO_FLAGS)
 	$(MMTK_CARGO_FIXUP)
 
 debug/$(LIBMMTK_RUBY):
diff --git a/gc/mmtk/extconf.rb b/gc/mmtk/extconf.rb
index 70244ae..3674204 100644
--- a/gc/mmtk/extconf.rb
+++ b/gc/mmtk/extconf.rb
@@ -134,7 +134,7 @@ def filter_warnflags(cc, flags)
       CFLAGS += #{cflags}
       DLDFLAGS += #{ldflags}
       warnflags = #{warnflags}
-      MMTK_CARGO_ENV = #{env}="-Clinker-plugin-lto -C target-cpu=native"
+      MMTK_CARGO_ENV = #{env}="-Clinker-plugin-lto -C target-cpu=native -Cllvm-args=-inline-threshold=$(MMTK_INLINE_THRESHOLD)"
       MMTK_CARGO_FLAGS = --target #{lto[:host]}
       MMTK_CARGO_FIXUP = mkdir -p release && cp -f #{lto[:host]}/release/$(LIBMMTK_RUBY) release/$(LIBMMTK_RUBY)
       endif