I've had a strange performance issue for a while now. Every now and then, I'll boot up my PC and Skyrim will run very well with an ENB, in this case medium quality Tundra ENB. I would get an average of 50 frames per second outside and 60 frames per second inside. But for some reason, I exit Skyrim, and the next time I play it, or if I reboot, it doesn't run well at all. My framerates drop dramatically, and I get mouse lag.

I've wondered if this was a driver issue, so I reinstalled the 320.49 Nvidia drivers, which didn't help. I also reinstalled my sound drivers, and uninstalled them completely, which didn't help either. My Skyrim performance would still be quite low. I also ran Skyrim without ENB, which restored my performance.

There was another nasty issue not just with Skyrim, but with other games where it would frreeze and stutter, and my sound would just go awry, causing me to restart my PC. With the reinstallation of graphics drivers, hopefully that fixed the problem.

But I don't understand why this is happening. Why would my performance drop, after being able to play several times with very good performance with ENB? Any help would be appreciated, thank you!

Might it be in-game weather related?

I don't think so. While I did have good performance, I had been to multiple places including Riften, which is intensive, been in different weather, all with good performance. Though at this moment, not anymore.

Try going in to the Skyrim Launcher, in options, try switching shadows from Ultra to High.