Network Speed

Since couple of weeks our Network speed has gone down, could any tell me what is the reason, we have not changed any thing on the network setup except installing the Lotus Notes Domino server, and lotus notes clients. will that makes the difference,from some of the lotus notes experts, I have asked, but they say, this is not because of Lotus Notes. So please some body be kind enough and give me some tips, how can i diagnose this problem.