It is difficult to find memory hogs ... and server load problems, as hardly anyone can reproduce the problems (I have php 5.31 and have problems installing your configuration)
I do not know the server load done by pppro, especially if you are running vb
As a matter of fact, I personally do not know if running vb
3.7 is a good long-term solution. (but that is another problem)
I am seeing a big gallery with 15,000 users and over 600,000 images.
The owner changed from pp7 to vbgallery 3.0 and did not run into any problems.
but I think that as long as you are running vb
3.7 there will always be optimization problems.
If I were you, I would try pp7 and see if there is a difference. (there is bound to be one as pp7 installs more or less outside of vbulletin)