In case you were wondering exactly which powerup, it's the Doomguy powerup. You need to get a few kills (unsure of the exact number, but I'd take a guess at 10-15) and then hit space to activate it, which puts you into first person. However, i have noticed a small bit of extra lag when you do it, which I imagine is simply because Unity has to render more shapes and in more detail than normal. So just be cautious of that. You can also come out of first person by filling up the powerup again and hitting space again ^-^
Indeed, the game may be laggy in first person, because it had to render the whole map.
It's not the priority to optimize it at the moment, but i will look for this later. By reducing the view distance with fog, it should be much better !