The performance impact depends on the way you lock. You can find a good list of optimizations here: http://www.thinkingparallel.com/2007/07/31/10-ways-to-reduce-lock-contention-in-threaded-programs/
Basically you should try to lock as little as possible, since it puts your waiting code to sleep. If you have some heavy calculations or long lasting code (e.g. file upload) in a lock it results in a huge performance loss.