This issue seems to have been fixed in newer versions of LastPass extension for Firefox, however as LastPass makes frequent changes, we have noticed that this issue has happened on a few of their updates. Unfortunately this is outside of our control. If you do notice a slow loading editor or delays when dragging and dropping elements, we would suggest to temporarily disable browser extensions to see if that could be a cause.
Also make sure your memory limit is high enough. While 256M is recommended, sometimes it is necessary to go higher than that if possible.
We have found some instances of the OptimizeBuilder editor loading slowly when using firefox LastPass extension.
This may be fixed in the future but if you are having this issue we recommend disabling the extension or using Chrome to edit OP3 pages if you experience this issue.