I've been playing Minecraft with Forge installed, but only using Optifine under it. I also mostly play in 1.10, so I never really noticed this problem before.
I use MultiMC, which may have some relevance; I'm not sure.
I switched down to 1.8 so I could play on a 1.8 server. This instance had no Forge installed, but did have the extracted Optifine jar, installed as a "Jar mod." When I started playing 1.8, I noticed I was getting maybe a ~250-300 FPS boost over 1.9/10. Naturally, I thought this was an issue with 1.9/10, so I did a ton of research on that, ultimately coming up with nothing.
Then I installed Forge on my 1.8 instance. As soon as I did that, 1.8 had the same FPS drop as 1.9/10. I uninstalled Forge and it went back to normal. I uninstalled Forge on 1.10 and it went up to almost what I get in 1.8.
The FPS drop isn't the only issue it seems I'm getting because of Forge, either. Without Forge, moving my mouse back and forth results in very snappy turns: my FOV moves very quickly and I stop right away; it's fluid, but precise. With Forge installed, it seems I'm getting some sort of microstuttering, as 128FPS with Forge is much choppier than 128FPS without. This microstutter might be what's affecting turning, as with Forge installed, turns are somewhat choppy, aren't precise at all, and seem to have quite a few dropped frames.
Since this happens on 1.8.9, 1.9.4 and 1.10.2, I'm thinking it's something to do with Forge, since 1.8.9 had an older version of Optifine without the dynamic lighting feature that was introduced for 1.10.2 and then 1.8.9.
Of course, I can just use Optifine without Forge, but I do want to install some other mods, like an armor status HUD. which only work with Forge. PVP is terrible with Forge installed, and I don't really know how to fix this.
Sorry for the coherency of this post. I'm not quite sure how to discuss this. I also don't know what I need to provide to potentially resolve this issue. I did read through the posting rules, but I'm not so sure how to apply it, since this isn't a Forge crash, but Forge causing major lag spikes (the Optifine lagometer is about twice as "high" with Forge than without). Please tell me what you need; I'll edit it in as soon as I can.