There is such a thing as root bound, and it affects ALL plants in the same manner.
If you can see roots at the outer edges of your root ball, you're about to be root bound.
THE ONLY SITUATION IN WHICH ROOT BOUND DOES NOT MATTER IS HYDRO/AERO/BUBBLEPONICS, for obvious reasons - you give the roots a chance to breathe after giving them nutes, whereas in soil things will get stagnant and eventually compacted with time and repeated waterings.
When you grow in soil, root-binding becomes a massive problem. I just had to transplant a Sweet Basil plant because of it being root-bound and requiring constant watering.
Also - this is definitely root bound bud. The uniform yellowing across the entire plant and not just older leaves is the giveaway - those plant roots need some aeration and space, ASAP.