RAID1 should not decrease performance in any way unless one of the hard drives is failing. If anything, it can improve read times.
Don't be tempted to run a CHKDSK. It can potentially destroy the RAID data patterns.
When you say speed has slowed down - in what way? Data transfer speeds, start-up speed, general usage etc? Could well just be you machine getting bloated with software, old drivers, registry corruption etc. It happens to all machines over time. I like to re-install my machines at least once a year just to tidy up!