Is it normal for the program to be quite slow at the loading screen and also when importing flightplans from Navigraph?

Loading takes a long time because 2GB+ of data are being loaded from disk.

Importing can take a while because of numerous factors.  If you turn off "calculate safe enroute altitude" option, you can at least avoid checking the entire route for elevations.

And of course, all this depends on what CPU you have and how much memory you have.

Dave

Thank you for confirming this is normal!