Blank pages, slow running time

Ever since I added in a couple of WPMUdev plugins - my website has been having problems. I installed and uninstalled marketplace because it was interfering with my template. Currently I have appointments+ running and love it. But my website is significantly slower and I often can't open pages. It just shows a white page on the screen. My team can't blog or do posts or change anything while this is going on. Have you encountered this before? Could the plug-in be causing this problem?