Hi,
Adding anything to the bin directory will cause ASP.NET to re-compile all of the assemblies. Depending on how many DLLs you are adding and what is required to compile each one, this re-compile can take a while. The contents of the App_GlobalResources directory can also impact this compile time. We used to ship all available language resource files with each release of mojoPortal. A while back we started shipping only English and Italian because they are the most complete. The others are available in a separate download. If you open up your App_GlobalResources directory and have resource files there for languages you do not use, you can remove them and that should help speed up the compile time.
HTH,
Joe D.