In my opinion, it is probably something else. Insufficient free disk space
can slow things down, but it is often the case that other influences are
more often responsible for slowness. (Including Spyware/Virii.)
If in doubt, check your computer's available disk space.
Insufficient RAM also slows things down, but your emails have nothing to do
with that. All those emails are not saved or stored in RAM.
Any email 'client' program that downloads the emails onto your computer will
use disk space for the emails. That includes Outlook, Outlook Express,
Thunderbird, and many others. Anything that uses POP3 or IMAP to get your
Gmail.
Gmail now provides 7GB of storage on their servers (not on your computer),
and it takes a LOT ... and I mean a *LOT* ... of email to fill that up.
I've received quite a lot of email over some years and I am nowhere near
7GB. Most modern computers come with hard drives that are much bigger than
7GB so it would be difficult to approach filling up the hard drive with
emails, unless you regularly receive truly enormous amounts of email with
attached files or pictures. I know you say "WAAAAAAAY too many emails," but
the diskspace they take up is probably not as bad as you fear. (Could be
wrong, though ... don't know what your definition of "WAAAAAAAY too many"
is, what kind of email you get, and I don't know the size of your computer's
harddrive.)
Andy