I've had all of these issues as well. Plus edits not saving, Firefox showing error messages when trying to load the back end of sites too. Site content not loading.
Having no custom code in sites makes no difference.
Clearing caches makes no difference.
Firefox, Chrome and Safari all having issues.
Having other tabs/windows closed makes no difference.
Having Adobe CC closed makes no difference.
Having fewer browser plug-ins and extensions makes no difference.
Working on an 8 core newish MBpro with 64GB RAM, a 1GB internet connection with newish super fast router and modem... yet all these lags.
This has been going on MANY months and is ruining my work flow and thus impacting my income.
Contacting support brings no real results. I get emailed support articles filled with info I already know and steps already taken.
Please fix this Squarespace.