nothing changed in recent versions of mojoPortal that would affect performance and there have not been any other reports of it being slower after upgrading. the last few releases have only been small maintenance updates.
slow startup is a very common issue with asp.net in general, the asp.net compiler has to compile all the .aspx, .ascx, and .resx files as it encounters the use of them in a request so when the files all get replaced during upgrades it has to do a lot of that. normally after visiting a page with each feature it should be warmed up and faster. all that compilation happens down below the windows folder in the temporary asp.net files
when the application has been shut down or restarted it also has to re-compile some things, for low traffic sites people sometimes use an external service to ping the pages of a site periodically to keep the application awake because by default IIS will shut down an application that has not received a web request in the past 20 minutes
you mentioned using the language pak, that adds a lot of resx files in the App_GlobalResources folder which is a lot more files to compile during startup. My advice is keep only the English files and the files for specific languages that you are targeting. Keep in mind also that adding or removing files from that folder forces it to recompile again and causes another temporary slow down.
another thing that can impact performance is if you have <compilation debug="true" in Web.config, that should always be false in production.
That is all I really know about that, but if you continue to have questions please use a new thread, this thread has already deviated way outside its original topic to a lot of tangential things.