I don't know if you even have the option of turning off the signs. I've played around with my settings, and I've only put them as low as around 50%, but the signs were still there. I think the effect the taxiway signs have on the game's performance is minimal.
They're everywhere! Yeah, the taxiway signs are so small that they have to effect on the performance. It's fun, I love taxiing to the runway now and looking at the signs instead of having to use the 'purple/pink' line!