This has significantly decreased page loading times and other metrics that are extremely important for the success of a website to attract and keep visitors. It looks like it was written with care and I have had no issues with it since the install. This is one of those addons that is fairly essential if you are concerned about your PageSpeed score. This should really even be part of XenForo. I thank the developer and give him kudos to providing it for free at this time.
What exactly does "Doesn't work with subfonter" mean - do you get any error message?
What were the exact steps you did take to install and configure the Add-on?