I'm getting a few thousand mails a day in three accounts -- about half of
them spam, the rest ordinary mail and a lot of mailing list mails. KMail
handles that just fine, including the spam filtering.
But I'm using pop: not sure whether imap would would work as well. My
archives aren't that big though, I throw away a lot of mailing list mail
after having skimmed it, and my archives don't go back to 1993 anymore
after a stupid crash.
And I'm going to try mailody soon, which uses Akonadi and is designed for
handling large amounts of mail over imap.