Sooner or later every application gets larger and more complex. Thus it is unavoidable that things starting to slow down a bit. When this is happening, it is good to know where the performance leaks are. This can by achieved by a good profiler.
A pretty reasonable one is the Eqatec profiler: http://www.eqatec.com/tools/profiler/
It is easy to handle, intuitive to use and spits out just the important data to pin down bottlenecks. I use it in several of my projects quite successfully and as an embedded developer I like the fact that it can be used across all .NET platforms ranging from server/desktop to .net CF (Win CE/Mobile) and even Windows Phone 7!
There is a free trial available from the guys in Denmark creating the tool.
Just check it out yourself and get it under your tool belt!