My own 4 x IMAP accounts (Gmail, Outlook, Yahoo & iCloud) account folders all sync pretty quick and no issues with more than 100,000 emails using iOS 26.2.1 . Nothing gets stuck.
So could be the account eg “had a connection error with the server” so stopped synching which might explain why it looks / appears stuck.
So press “Menu / Settings” and first look for any “Red errors” next to any of your accounts. If there is errors close and reopen eM Client.
If no red errors next to accounts, then press “Show Operations” and see if it’s still initially synching, as could be there maybe acct folders “still not all initially completed yet” since setting up the app.
Also if you have a Gmail account, initial downloading / synching messages to folders time might also be caused by eg: Gmail throttling as @Michal_Burger advises in his post below.