This is a problem in the hosting machine where the site is located. Apparently low-quality hosting doesn't withstand a lot of user traffic, which is why the site loading speed is lost. I've been using the services of
Mangomatter for a long time and on my sites and forums I haven't encountered such problems although all are loaded on one hosting machine. You need to be very painstaking about choosing a hosting service for your site, since its performance depends on it. If you see that your site isn't developing and is standing still, try changing your hosting and you'll see how user traffic on your site will increase.