Mail applications get very bloated. Why? Because they try to index ALL your mail on your device, and you probably are accumulating more mail than you actually plan to search through.
Instead of tidying up, people just archive or keep everything. And the poor email application doesn't realize that you don't really care about all that mail but it indexes it anyway. Over time, it accumulates gigabytes of data and indexing that takes time, computer power, and drive storage.
If you have problems with your mail application (typically Mail.app on the Mac, Outlook on Windows), your FIRST step should be to try webmail as an alternative. If it, too, has issues, then your local application may actually not be the problem: it may be with your account (i.e. outside of your computer and at your mail provider, typically Google Mail or Microsoft 365)
While you await technical support, use webmail. Mention to the IT person "my account is working on webmail without issue" - this will help them target the issue as more local and less likely to be something that needs to be fixed in the cloud.
While you're getting by on webmail until your main application is fixed, consider using it exclusively. Why? Because webmail does nearly all its indexing on the server side, delivering the results much faster to you than if your computer had to do it solo. Ask your IT professional what application they use: you may find they use webmail all the time. It is extremely lightweight, and can handle an unlimited amount of mail because it's not storing it locally on your computer.
Were you born before 2000? If so you might say "but I need my data with me in case I'm off line" - most people born after that are nearly never offline, so this is really more of a psychological barrier than an actual limitation. With 5G, fast ubiquitous connectivity will just accelerate to be the norm.
Try webmail when your mail application is "in the shop" - then consider keeping the loaner and just using it daily.