I run on 1.6.4 and the game is super laggy, and with OptiFine I can help the lag. However, I want to know if it'll be acceptable to use it because it could be considered an advantage to have higher FPS and a zoom option.

They say that it's allowed as long as you don't use the zoom

Optifine is allowed currently as long as you dont use the Zoom function. There is a problem however of Optifine playing a role in how, for instance, end portal eyes generate as well as other world generation. So it is currently up for some debate.

There are confirmed effects that Optifine settings have on world generation, so some people think it could potentially be banned/restricted, but it's in ongoing discussion. You may use it right now, as long as you don't use the zoom function.