I mentioned about that recently in this thread:
https://forum.emclient.com/t/em-client-needs-4-6-minutes-to-start/80156/79
In my opinion local database managment in eM is totally basic and not appropriate for this kind of application. It’s extremely slow and not optimised for performance at all. These problems were solved with most of e-mail applications somewhere in the late 90’s - eM client is still not there. With small database files these problems are invisible, but as the database grows larger, performance becomes a real issue. Problem does not exist with IMAP as there the database is managed by server.
All operations on local databases or with POP3 accounts become unusable at some point. Opening an attachement, moving messages between folders, even replying to emails etc. My eM-Client, with less than 1GB database and approx 20k archived emails now needs almost minute to open, moving a message between local foders sometime freezez app for 20 seconds. And we’re not talking about some 10-year old budget laptop but a modern powerfull workstation PC used for serious work. Looks like eM-Client needs to re-index the whole database on every startup with some poorly optimised algorithm.
I have another license without local database (only IMAP accounts) on my second PC and it works perfectly fine.