Slow start up saying 'optimizing app 1 of 1'

[AC Question]

My android has just started to have a very slow start up saying optimizing app 1 of 1. What is this app and why is it taking so long to optimize. I will uninstall if I knew what it is.
Thanks.

 

[Rukbat]

It's not an app, it's Android. the Dalvik cache was cleared, and the phone is rebuilding it.