Not a hacked client as others have pointed out, in fact it has a built-in anticheat system. Many people use it on Hypixel given that it's a specialised client for PVP. As for FPS improvements, they boast their own FPS improvements, which is called 'BetterFrames'.
I doubt that Badlion actually has Optifine, or it is infringing on sp614x's license, which forbids distributing it even in modpacks, much less as part of a custom client (which is really just a mod/modpack as otherwise it would be against the EULA to distribute; the actual game must come from Mojang's servers, which is then modified with separately provided files which are useless on their own). That's not to say you can't replicate any of its features with your own code (for example, I added a zoom function to my own mod, which is nothing more than a FOV toggle, "better grass", which uses the top texture on the sides (Optifine's version is a bit more complex, taking the surrounding blocks into account), FPS and render distance sliders (replacing the several fixed settings in vanilla 1.6.4) and there are also many other optimization mods which may or may not make similar changes as Optifine, and even many features in vanilla used to require Optifine).
Yes, it doesn't actually come with Optifine. The client just helps to download it for you. So it's more like Optifine-compatible instead of having Optifine pre-installed. It's probably still against sp614x's license though.