I think it's a large part of the issue, but not the whole. However if you look in the world builder lots of the in game textures are actually made from splats and splines leading to huge performance issues, whereas on DoW II you could have 8 different painted textures, in this I believe you can only use 1, correct me if I'm wrong. This means if you want road surfaces or paths you have to use splines, so maps have excessive numbers of these in CoH 2.
You can use more then 1 color to paint the ground. You can paint roads with it, if you want to , BUT it will look awfull and you dont get red cover or speedbonus for vehicles.
Splats and splines are needed for any map that is supposed to look and play "good".
Tbh it has been a while since I was working on a map, but you can quickly and easily fill the map with splats and splines. This can affect the performance because there are also specialeffects tied to some of those ( like the little dustclouds on some roads, for example).
The things that realy make a BIG difference in performance are flames and smoke + to a lesser degree all those "little dustclouds" you can see on every step a soldier makes or when vehicles/tanks drive around. The sad thing is all those specialeffects make the game/map look good, so you want to see many of those when playing, the result is bad performance.
I am not a programmer so I don´t know if this could realy be optimised, but mappers could do every player a favour with just using "specialeffects" where they realy are seen and make a map look good while you are playing it.
Sometimes less is more.
A lot of the official maps and workshop maps could look just as good or better with half the splats, splines and fire, etc. used, if the mappers would have put more effort into the lighting, fog, wind and the transitions.