Help.

[LookingForAStar]

Okay I downloaded a new realistic mod and now my game is so laggy, it's unplayable. Please help.

[AlphaEclipse]

Sometimes mods will do this. The best you can do besides redownloading FH would be to go into your ganes settings and turing off water reflection, and other video options. This may help a bit.

You can also see if changing you subrendering system in options helps.

[Vespian]

What kind of mod did you download? The mod should have a key or key combination that activates/deactivates. For example, Red's ENB mod activates/deactivates by pressing the Shift + f12 keys. If you have a mod like an ENB mod, you could try deactivating the mod so you'll have much less lag.
Also, you could try turning off your water reflection and lowering your view distance. That'll also reduce the chance of lag as well. If that doesn't work, try changing rendering systems like Ms. Eclipse has stated before me.

If all else fails, you may have to re-download the game.

[LookingForAStar]

I downloaded Brokami's realistic texture mod to make everything look realistic.

[Ruby1234]

Your computer cannot handle the textures.
You can try shrinking some of them, since some are over 4000x4000.
Other than that, just install FH again, and it'll replace them with the defaults.