After a fresh install of Loki without making any modifications the wingpanel takes a really long time to load and often stops responding. Sometimes when it doesn't stop responding the OS ceases to respond to mouse clicks. My issues with Loki have caused me to go back to Freya as Loki is currently unusable for me.
I regularly notice a heavy fan activity when software is updated from within the appcenter after the system has booted, combined with a retarded system response to mouse klicks and keystrokes in other programs like browser etc. From the shell, when I run the "top" command, I can see wingpanel consume a lot of CPU. It seems that wingpanel is caused to loop somehow.
After the updates are done, I note wingpanel's PID, terminate "top" with CTRL-C, and "kill " wingpanel. The system then gets back to normal (quiet fan and a hardly visible wingpanel in "top").