Is it better to put all the Business Layer and DB layer classes in one solution

[Niccu]

I have a project which is built on 3 tier arch.
UI, BL , Dl. BL and DL are in one solution. As there are lot many forms etc, size of dll is becoming bigger and bigger.
Is it going to affect the performance? If "yes" what will be the optimal way of doing the things.
Regards
Niccu