Thanks for reaching out to the Community! We are currently tracking this issue, and I have tracked this on your behalf. The slowness you mentioned appears to be happening later in the evening and at night, and only with 2GE Legacy templates. The current and only workaround is to switch to our 3rd Generation Editor (3GE).
Here are a couple of things you should know:
If you would like to get a head start on moving to a 3GE template you can check out this article on Switching to the Third-Generation Editor from the Legacy Second-Generation Editor or give our Support team a call!