I think the recruitment/XP/training system is probably okay in itself. But bigger-picture, different aspects of a game interact and affect other aspects in unintended ways. I perceive this troop recruitment/development model is somewhat over-stressed by the incessant factional warfare of Bannerlord....which leads to the AI lords stripping settlements bare of all troops regardless of tier. Part of the reason that these guys are running around with all-recruit/peasant armies is that's all that's available in the towns/villages. Another factor is the current quick-escape issue, where every Calradic lord is a Houdini. They're never "off the board" for more than a day or two, so ALL lords are running around putting intense pressure on the settlement troop sources; probably more than TW had visualized or foreseen.
Many of the various recurring issues in the forums, like this one, or economy, or garrisons and food-vs-prosperity, or the discussions about execution penalties, etc, I kinda see as symptoms of a more fundamental problem, rather than separate problems in and of themselves. That fundamental problem is the never-ceasing warfare. Quite simply, a faction is never at peace long enough to "catch its breath"; rebuild parties, develop settlements somewhat unmolested other than pesky bandits, allow the economy to prosper for a while, etc. This situation is somewhat distorting to some degree how players (and maybe even devs) are seeing various aspects of the game.
I suspect two very simple fixes would go a surprisingly long way toward helping a whole host of seemingly-unrelated issues:
1. Make captured lords stay captured for a while. Not forever, but roughly as long as they did in Warband; maybe a few escape or get ransomed quickly, but others might stay behind bars for the duration of the conflict. As the war goes on, this would cut down on the number of parties running around raiding, sieging, and recruiting townies/villagers.
2. Impose some period of inviolable truce when wars end. SO many times now, I've seen an Aserai-vs-Vlandia war (for example) "end"...and then another Aserai-vs-Vlandia war flares up again just a few days later. Areas around Charas, Ortysia, and Quraz never get any respite. That doesn't necessarily mean that a faction will have many long periods of overall peace...but at least it will serve to distribute hotspots around the map a little more, and certain areas can sometimes get some more "local peace".
Neither of these are new or complex mechanics; both existed in Warband.
Just to explicitly tie this back to the OP topic: if the overall pace and intensity of factional warfare eases up a bit, then the current process of recruiting and training up troops might not feel so slow in comparison.