Even if he was a spy, which I don't see any proof saying he was, it doesn't explain the laughably bad casualties for Russia, while the Germans barely took any
It was a joke, he wasn't spy. Rennenkampf was loyal to Russia and Russia wasn't nazi to deny foreigners to high positions. He wasn't even foreigner, Russia had millions of ethnic Germans in the Russian Empire alongside other 1000 nations, nationalities and ethnicities.
Germany had four fronts, Western Front against UK/France, Eastern against Russia, and also sent troops to the Italian front and the Austrian front against Russia and Romania
Austria had four as well, against Italy, Serbia, Russia, and Romania
Helping Austria wasn't exactly a front for Germany. Germany had 2 fronts, Austria had 4 (italy, serbia, romania, russia) but still nothing compared to Russia.
Russia had to fight great powers like Germany, Austria-Hungary and Turkey, all large strong empires with technology and manpower... (turkey imported german tech)
Austria fought little Serbia which barely left two balkan wars still not recovered and replenished reserves, and they had help from Bulgaria, then when that front was over, Italian front was opened, and after a year Romanian front was opened. The only constant front Austria had was Russian. While Russia constantly had 3 fronts with major powers.
The greatest strain was put on Russia, then Germany, then Austria. That's why using all this facts; Russia is rated very good in WW1.