The PMDG aircraft is optimised out of the box, but this is aimed at the end users, who often have low specs and could use a little more headroom with reduced texture resolutions.

The gain of FPS is not thoroughly calculated however any FPS gained is better than not. Note that if you have a very low-end CPU / GPU or already have very low performance, this mod will most likely not help.
For me, with the PMDG screen frames set to 5 FPS, I got anywhere between a 5 to 10 FPS gain.

For all users:

Locate your MSFS 2020 Community Folder then the following directory:
pmdg-aircraft-738\SimObjects\Airplanes\PMDG 737-800

Make a copy of texture.common and texture.vc should you want to revert at any point and then replace the two folders with the two found in the downloaded zip file