I have thousands upon thousands of text/picture messages on my Samsung Galaxy S 2, ice cream sandwich. When I go to open a conversation, the phone stops for a good 30 seconds, even after a fresh reboot. I don't want to delete my messages, but I'm sure if I could somehow limit it to, say, the last 100 messages by default, it would load much quicker. Again, I don't want to delete them, and I still need access to most of the messages, but only on demand
Try using an alternative to the stock SMS app like Handcent SMS. A cautionary note - turn off the auto delete option (the option to auto delete messages older than X days).
An additional note - since your SMS store seems quite precious to you, I'd also recommend SMS Backup & Restore. It can backup your SMS at scheduled intervals and also email / upload them to Dropbox if need be.