[Smush Pro] Site speed

Before I installed Smush Pro, I ran Feel-Good Teaching through Google Page Speed Insights. It came back with a score of 53, with quite a few issues stemming from my images. Not good!

So I bought this app, installed it, ran it and re-checked Google, and now the score is 35. Why would using Smush degrade the score on Google and how can I fix this?

Thanks for your assistance,

Kerry