>>715698195*All* frame gen adds input latency. It literally can't work without it.
The way frame gen works is it delays your display output by 1-3 frames so that it can generate fake frames and show them to you first.
The problem with Lossless Scaling is that it's frame gen is at minimum a 2x, meaning it will add a fake frame between every 2 real frames. So, half of your display output would be fake frames. It sounds like you want just a small extra boost in fps, so you'd probably want something like a 1.25x frame gen (currently unsupported afaik). It would still give you input delay, but it wouldn't be nearly as bad since only 20% of your frames would be fake compared to 50%.
But, it also depends on what kind of a frame drop you're experiencing. If it's caused by a CPU bottleneck, or if the game logic is tied to the render loop (it is in many games), frame gen can't help you with that. Your game will still stutter.