This change does not slow down fighting speed. It does slow down auto battle clicking, which is not the same thing. The change to the available units is great, and the very minor initial inconvenience of actually having to pay attention in between battles is actually an improvement to the game. Maybe once the auto-clickers slow down a little bit, they'll learn how to actually play the game.
When one is racing for a GbG province it's (at least sometimes and I'd suggest usually) helpful to auto-click fights to get as many advances as quickly as possible to take the province (especially on GbG opening day). As attrition increases it can be helpful to customize your army units based on what the game puts against your army.
When the units were grouped by type (before the 1.271 update) it was faster to be able to click the top left unit X times to clear the units you want to replace and then add the units you want to use. Now, you have to move your mouse or finger or whatever to remove the undamaged and damaged units of the same type (which takes more time) and replace them with the new units.
Having used the new system for a few seasons, the new organization seems to slow down fighting speed due to extra pointer/cursor/mouse movement/clicking -- it may not be by much, but in a close race it doesn't need to be by much.
I'd prefer to see that change as a toggled preference like the unit stacking.
