Speed up my slow multisite network.

The entire network multisite is unbearably slow that I am considering moving all sites to standalone versions because I am getting nothing but complaints from customers that the front end and the admin sections are slow as death. I need both admin and front end to be really quick.

Other sites on my server run fast on both frontend and backend.

But in this, I am having some issues for example:

I deleted my debug.log about 10 days ago, so it started fresh and I could monitor it. I have just exported the debug.log after 10 days and it is at 2.2GB. That surely isn’t normal and there must be major issues with my multisite network that I can’t figure it out.