Email slowness can be down to many larger files still in the inbox instead of archiving. Also the servers from your provider if a smaller company, can put a brake on technically to allow other users an equal service.
I have Outlook and Gmail which can slow down occasionally if I have files with links to advertising or news or downloading.
Servers like Landyzone will read the traffic flow and I suspect it kills the speed with the downloads of advertising and file we send. Try clean the cache to remove dead files and temps.