Vis a vis Janis (or any other receiver) "grasping the playbook", it's not just memorizing the base routes in the calls.
You have to be able to read the defense the way the QB reads the defense, the so called "being on the same page" or the QB "throwing the receiver open". The QB has to have confidence the receiver is going to the point where the QB thinks he's going before the receiver breaks. At the highest level, the receiver calls for a change in route with a look or hand signal at the LOS that the QB confirms with a look or hand signal, or visa versa with the QB initiating the change and the WR confirming.
Whether it's an option route, reading a blitz, reading zone, reading corner inside or outside technique, press or off, or adjusting to the depth of a safety or an ILB, every route has a subtlety. There's a lot of sophisticated playground that goes into it. Making the right adjustments or choosing the right option is Job #1. Assuming the guys come to the project with decent physical attributes (catching fundamentals, hands, quickness, speed which is more or less a given at this level), he who masters the subtleties of route running wins snaps. A couple of tenths on a 40 time or a couple of inches on the tape will not matter.
So far, Janis has shown himself to be more athlete than wide receiver or even football player. But you never know. It took the better part of 4 years for Perry to be a guy.