I thought I'd give the free version a go before getting the paid version, but I did go ahead with this after seeing the results.

I checked on GTMetrix before adding it, and it really didn't look good. I added the plugin, and with no changes to the settings, ended up with the following:

BEFORE
Performance - 34%
Structure - 86%
Largest Contentful Paint - 3.6s
Total Blocking Time - 424ms

AFTER
Performance - 100%
Structure - 98%
Largest Contentful Paint - 429ms
Total Blocking Time - 0ms

My Google PageSpeed Insights score is:

Mobile - 99
Desktop - 100

It does take a while to go through everything. I'd suggest leaving it 24 hours to see the full benefits as testing as it is going through, like I did, will give you a confusing collection of good and bad results.