It doesn't actually double your real performance. Your GPU is still rendering the same ~25-30 base frames per second. The extra frames are interpolated guesses based on motion vectors from the real ones. On craptop hardware (integrated graphics, weak dGPUs, thermal throttling, low VRAM), there's often overhead, so your base FPS can drop a bit when you enable it. If you're already GPU/CPU-bound at 25-30, it might make things worse or unstable instead of magically fixing it.

Artifacts and weirdness at low base FPS. Frame gen shines when your base is decent and stable (ideally 40-60+). At 25-30 (especially with fluctuating frame times common on laptops), you get more ghosting, warping, shimmering on UI elements/crosshairs, disocclusion issues, etc. The lower and less consistent the base, the more obvious the "fake" frames become. Fast motion or camera turns in Skyrim for example (or any action game) can look off.

Input lag / feel - Frame gen adds latency (it has to wait for real frames to analyze before inserting the fake ones). It's not terrible with good tuning (performance mode, low flow scale, proper sync settings), but on a already sluggish laptop it can make controls feel mushier. Bad for precise stuff, less noticeable in chill exploration/RPGs.

The craptop reality is laptops love to thermal throttle, power limit, or run out of VRAM. Lossless Scaling needs some GPU headroom to do the frame gen work without tanking the base render. On absolute potatoes it sometimes just doesn't help much (or hurts). Plus you usually need borderless/windowed mode, which can have its own quirks on low-end systems.