smacking the phone actually works! I've been trying to get out of the bootloop for the last couple of hours... I tried about everything suggested here but nothing really seemed to work.
Although I noticed that constantly smacking the phone on the table (not to hard) during boot gets it further in the booting process. I've kept on doing this all the way through the "optimizing of the 136 apps". I've probaly chucked it 300 times on to the floor
After this, it actually booted, but after about 5sec not hitting it, it powered off again...
I got really fed up with it and threw the phone pretty hard on to the floor (with its spigen cover...) and guess what... it booted stable!
So: hitting the power button pretty damn hard actually is the solution! For now...
Thanks you guys!
the happy owner of a fully functioning Nexus 5