Since you have 16GB of RAM Windows ReadyBoost will be hammering your HDD like crazy during game play.
- It may even be the Indexer/Search service(s)
Slap this in your StartUp folder: http://scottdbowen.id.au/PreCacher/ (from 15KB).
Restart your PC three times waiting no more than 5 minutes between restarts.
On the 3rd restart wait until PreCacher has finished running.
It will retrain the ReadyBoost traces if you have it enabled to concentrate on the 50% to 80% of your smallest files on C: drive instead of looking elsewhere and prefetching large texture data during game play.
Load either game again and let me know if it helped or not.
Failing that I'd give the IObit Smart Defrag or Diskeeper tools a shot, since they can both perform better full defrags much faster than the built in one that Windows is bundled with. (Any of the three can use the ReadyBoost data for hints I believe).