It’s actually a bug in the Shadowplay, not in the Win10. I’m at 8.1 currently and the same problem happens unless you set Shadowplay to manual mode. Shadow mode works by continuously recording games in the background. To save disk space (video takes a lot of space) it only preserves last X minutes (up to 20). This in turn allows to save something that happened in game recently (great dogfight? some funny bugs?) with single hotkey without even previously starting any recording. So it starts recording at the menu, in 1024x768, and keeps that resolution and aspect ratio (cause it’s continuously recording) even after the game switched to whatever other resolution. In manual mode it’s on-demand recording and works OK.
Shadowplay barely causes any framerate hit on modern nVidia graphic cards, series 600 and onwards. It achieves that by using hardware encoding - that is, a special chip or something mounted on the graphic card - to do all the encoding, rather than putting stress on CPU or other GPU parts responsible for real time rendering. Hard drive speed was always needed for video recording, or even better, a separate hard drive, one without operating system or the game being currently played.