Email is extremely slow to load.
First “check that your Internet connection speed is still as it should be and not gone slow”. A good site to test your speed is https://www.speedtest.net/
Also “check your mail server website incase there is any current outages” possible causing slow mailbox access which sometimes can happen.
Next suggest to “try also rebooting your computer” just incase there is any current resident programs possible in memory that might be interfering.
Now if your internet connection speed is normal and no reported mail server issues and rebooting your computer made no difference, then “see the following thread for different options you can do in eM Client” if emails are slow to load or slow to download or have gone slow all of a sudden.
Apart from the above link info, next try uninstalling eM Client and if you have Windows “don’t delete the database” when asked on the uninstall wizard. Then reinstall the same or later version of eM Client either from the Windows Store if you originally installed it from there or reinstall it from the eM Client website via the release history page if you originally installed it from the website. Then see if that fixes the issue.
Note: Before uninstalling and reinstalling, make a manual backup of eM Client via “Menu / Backup” incase you need to restore for any reason. You can see when the backup is complete in Show Operations via clicking the drop-down on the right of Refresh top left.
Lastly sometimes “optionally installed programs” can sometimes cause interferance with mail clients and cause slow mail loading or slow mail downloading such as eg: optionally installed firewall / security programs or optionally installed antivirus programs or VPNs. So if you have anything like that installed “other than what comes default with your OS”, then try completely disabling those to test.