I wouldn't think it's a matter of "performance" if memory is too slow the next instruction won't get read properly for the CPU and it'll freeze/never even start up. So as long as it's fast enough to work you'll be fine, it's not like the CPU would end up slowing down and affect "performance" like you're suggesting, it just plain won't perform at all.
The NES is operating at ~1.7Mhz and thus ~590nsec period. Assuming that phi2 has a 50% duty cycle that means PRG /CE is low for half the period which is ~290nsec. That means the ROM has about that long to get the data on the bus. You've got things like clock jitter and skew going on as well so it's hard to give an exact number but I would stay sufficiently far away from 290nsec. Your ROMs are probably even faster than listed also under ideal conditions as well.
So 200nsec is well within that and should be fine. IIRC I've seen games on bootgod's DB with speeds as slow as 220 - 240nsec which obviously didn't have a problem either. But I wouldn't push it much slower than that. Under ideal conditions like good 72pin connection, and cool atmosphere I wouldn't be surprised if 300nsec memories worked if you had some that were actually that slow. But I certainly wouldn't count on em
I'm just giving the numbers and theory here but someone else may have some actual testing to give you as well.