I started using OctoPrint with a RasPi 2B and OctoPi 0.14.0 and OctoPrint 1.3.4 but switched soon to a RasPi 3B because booting of the 2B already took quite a while.
Do you have plugins installed and have you tried to print in save mode?
The serial interface and the data transfer and the printer board calculation power are some kind of a bottleneck.
Besides the printing duties, all serial protocol calculation has to be performed too.
For more information on this topic you may have a look on this thread: The ongoing USB Conspiracy Theory?