Site speed improvement not showing on Google PageSpeed Insights

hi, I”m doing a trial of Smush Pro to see if it will have a big effect. After optimizing a few times, there was no real change in my score on the Google tool – I’m at 19 so there’s room for improvement – is there any sort of lag here? If I fix or replace the images Smush says it couldn’t fix, will that fix my score (and speed)?