I would say that you have a number of things that you need to do to first isolate/identify the cause of this.
1.) Upgrade to DNN 4.9.3, many of the updates in the 4.x series were for performance and stability. It is best to take a baseline measurement and do any diagnositcs when you are on the most stable, and in my opinion the best performing version.
2.) You need to look at your performance settings. if cache is set to Memory you will notice higher memory loads, my sites at time use 500Mb each, so 300mb for a larger site might not be that uncommon.
3.) Look at the load of the site, and the code that is running, how long are things going. Is the scheduler running long time etc.
4.) Contact the vendor and work with them.
I use Snapsis CSS NavMenu on a large number of sites, and in ALL cases we have noticed BETTER performance out of the site, and less resource usage.
If you need help with the diagnosis, feel free to e-mail me and we can talk msellers@iowacomputergurus.com (Without e-mail notifications here I can't always check back in)