The game isn't too appealing visually and mechanically, I am assuming but it seems that you are using Unity store assets and packs rather than your own ones?(they tend to have high poly and tri counts)

The design is overall inconsistent, what is with the cauldrons/campfires? what are the point of the hunger and thirst systems?

Also being a Unity engine game, this should not lag even close to as much as it does here.
Reasons why it might be like this:

You may need to do a double check on the poly and tri counts of your models and assets, having the numbers too high is diminishing your game and will only drop frame rates massively.

You have way too many unnecessary particle effects too the point that it doesn't look very good and also seems silly.

I feel as if there was a major lack of garbage collecting done with the data in this game, I have a beefy PC and decent internet yet this ran at around 10 fps (maybe less).

Even though, I can see that you put time into this, I think this game needs a LOT of polishing and maintenance but there is actually quite some potential here, I like the weapons system and; your fps controller is well done.
If you were to work on this more and add/fix the things that I mentioned, you would have at least a 4 - 5 start game.
Good job overall mate!

The FPS in this game is so abhorrently bad that it's practically unplayable.

From what I have observed by looking at the floor in the game to be able to write this review (due to it slowing down the entire web page): your health doesn't seem to decrease at all, there's a hunger/thirst system which has no sense in a game like this and the map design appears to be boxy and unremarkable.

There really isn't anything that distinguishes this game from the other hundred or more generic zombie shooters on the Internet.

It would be good if it didn't run at 1 fps.