Bitcoin also has a tendency to lag up after ~400 transactions in 48 hours.
Multibit locks up after ~150 transactions in 24 hours.
Electrum is the one I've been messing with for a while and so far the worst thing is the seconds not showing up for the transaction time. It's frozen up a few times, but a simple switching of the server fixed that.
Sounds like you're running on slow hard drives, or your hard drive is having problems. I was playing with a custom SD Bot using stock bitcoind on my VPS (Raid 10 + 10k drives). Lag didn't start becoming an issue until the wallet hit ~250 megs with 10k+ transactions. Even then it normally wasn't bad. Although now I have it modified to send change back to the original address, which stops the wallet from bloating up so badly in the first place.
My laptop is so bad (if you look only a few post back), you can see that I had to ask for help to get some Bitcoins from a wallet because of my computer's time being out-of-sync.
Bitcoin also has a tendency to lag up after ~400 transactions in 48 hours.
...
Is that a GUI issue?
I use the daemon, not the GUI (on linux a Core2 Duo E4600 2.40GHz with only 2GB RAM)
Runs for ages - and since I last restarted it (18 days ago) I've done ~900 txns