Works great - when it works. In fact, I was really excited about the concept! But most of the time, it didn't work properly. Some of our apps just did not play well with Hyperspeed. As regards speed improvements; sometimes I'd get as high as a B score on GT Metrix. Although often it would sink down to a D score. After I removed the app, and reloaded a fresh copy of my theme to remove all traces of code left from Hyperspeed I ran another speed test. I'm presently getting a solid C score on GT Metrix without Hyperspeed. So I'm thinking we can get by fine without this app.
Thanks for the feedback! Comparing individual audits can be deceptive since site audits can be very variable. The same site can have very different audits with no changes. Taking an average over time gives a better picture of how speed has changed.
Just let me know if you decide to give the app another try and I'd be happy to help!