Lucky1:
This usually happens because eM Client is using the legacy Exchange Web Services (EWS) protocol, which Microsoft is actively “de-prioritizing” (throttling) in favor of the newer Microsoft Graph API .
eM Client actually has a modern sync engine (OAuth2 / Graph), but for many Exchange/Office 365 accounts, it defaults to the old, laggy EWS. You can force it to use the faster API by editing the Windows Registry
Does that reg mod actually work for you ?
As eM Client doesn’t currently support Graph Protocol yet in release versions untill Microsoft fix its issues and EWS is deprecated, so that registry mod may not make any difference atm.
See @Michal_Burger post below.
We are now revisiting our support of MS Graph, but it still has several issues on MS side that they need to resolve to be a real alternative for EWS. We are working on some workarounds, but we need some work to be done on MS as well. However we’ll be ready for the switch the best way possible, I can assure you about that.