Apps open occasionally in meetings:
email
conversations
browser
calendar
xjournal or an texteditor

I hit 1-3 as this is for meeting times

normaly I got conversations & calendar open

I guess calls do not count...

for performance:

If you want to use your phone don't take a picture ahead as trackerd or thumbnailer will eat your device.
It does not matter how many apps are open, if only one of it likes to hug the mmc you are done with it... no response for minutes... so it is not only the RAM as many people claim, sure low amount of RAM and constantly swapping makes it worse!
I guess if one of us is looking into hardware details someone will come up with a pretty good idea for a hardware-setup to prevent the device from freezing while transferring files to ext. SD... improve the data pipes!