Well CQB is kind of like an ambient unit spawner so yeah. It adds additional units beyond the ones spawned by the objective modules. Depending on the percentage and numbers you let spawn with CQB, it in itself can bog an already crammed mission down quite a bit.
I prefer to never exceed 10% and pairs. On maps like Altis you need to be pretty mindful and probably use less if you allow them to spawn on the entire map (depending on how everything else is set up).