I've seen it from time to time, and have yet to move to the CSSU. It's usually caused by not having enough memory to run/load the apps. Check whats running in the background, what apps you have installed, and how much data you have in each app. Modest and other apps generally were not designed to hold thousands of emails. They're meant to do quick checks on the go.

That's not saying some of the CSSU changes may be causing it to happen more often. But it's not something new that just started with it. It's been there all along. I find often that rebooting and/or cleaning out old emails/sms/IMs tends to fix it most times.