Board Thread:Support Requests - Getting Technical/@comment-27098914-20200426132401/@comment-9605025-20200427031525

This seems likely to be the same as the bug reported here. However, please your specific case to Fandom as having more information may help them track down the issue.

As for the speed issue, it is probably because the "source editor" is really just a source mode within VisualEditor. However, it wouldn't hurt to report this to Fandom as well.