Optimizing a WordPress site can be a tedious process that involves improving the overall performance in a few optimization cycles. As WordPress plugin developers, we have dealt with hundreds of sites which included a large scale database with many installed WordPress plugins, and have managed to improve their performance as measured by the GTmetrix level.
After we were done, the overall rating had jumped from D to E or A to B. This was also reflected in the time it took to load the website, which we view as one of the most important metrics in the optimization process. Basically, any optimization will improve your site speed, but the more different tacos you use increases your chances of improving site speed exponentially.