What version of Go are you using (go version)?

go version devel +91a5f16ce2 Sun Jan 29 22:47:27 2017 -0500 linux/amd64 (roughly Go 1.8)

Currently the garbage collector scans the pending finalizers queue during mark termination. The length of this queue is under user control and also never shrinks from the maximum number of queued finalizers over the life of the process.

The STW time on my laptop is about (20ns * Q + 1ns * (M - Q)) where Q is the current number of queued finalizers and M is the maximum number of finalizers that were ever queued. These constants are quite small, but the following ridiculous example demonstrates how to use this to drive up the STW time quite high:

package main

import "runtime"

var sink interface{}

// active controls whether finalizers should be left on the queue or
// not.
const active = true

func main() {
	// Create a lot of objects with finalizers.
	objs := make([]*byte, 1000000)
	for i := range objs {
		obj := new(byte)
		runtime.SetFinalizer(obj, func(x *byte) {
			if active {
				select {}
			}
		})
		objs[i] = obj
	}

	// Release all of the objects.
	objs = nil

	if !active {
		// Clear the finalizers queue.
		runtime.GC()
	}

	// Run GC.
	for {
		sink = make([]byte, 1<<20)
	}
}