There has been much written about this.
The purges had a serious effect. Besides the strategic coup (Red Army in transition), the Red Army was pretty much a 'paper tiger' that accumulated men and equipment but not training or develop formation capability. It was actually combat experience that eventually gave them a better command cadre as the pre-war cadre was filled with incompetents. A lot of the vintage 1941 commanders did not rise, but operational commanders like Konev, Chuikov, Malinovsky and tactical commanders like Rybalko, Kravchenko, and Bogdanov proved themselves through successful command.
It wasn't until the winter of 1942 where the Soviets fielded the equivalent of armored divisions that could attack in depth. So they were a tactically impaired army that relied on mass for a long time. Overall, though, they got the defensive part of their calculus correct in doctrine by 43', and improving offensive skills was the harder part.
Reading 1944, 1945 operations is like reading about a totally different army than 41 or 42.
When the war began, the soviets lacked experienced and skilled officers in the higher tiers, due to the great purge. The Germans on the other hand, had alot of those. During the course of the war many of the "good" Wehrmacht officers retired had to retire or got killed while the soviet staffs gained more and more experience and learned from their enemy.
I think this point gets neglegted way too often.
Yep, the army was ground down and watered down due to attrition among other things. I enjoy reading operational history, and it is interesting to see that the Wehrmacht in operations increasingly became reliant on the actions of a few elite divisions, while the rest of it didn't have much punch and were good only to hold ground.
Of interest is the death of the high reward move, the 'encirclement'- this was largely caused by lack of infantry combat power and numbers.