The Wehrmacht was in no condition to invade anyone immediately after the Invasion of Poland. Poland was one matter; a nation with very few armored divisions and a small and obsolete air force was easy prey to Germany's new tactics. But the German generals needed six months at least to build up their forces enough to take on France, a more industrialized power with decent firepower and defenses, and were terrified that the French would attack them when they were still resupplying ammo and replacing losses. When the French did nothing but build up their defenses just like Hitler said they would, the Germans became more confident in their Fuhrer. Plus, they were still stockpiling resources from the USSR in preparation for the takeover of Western Europe.
Attacking the USSR at that point would have been nothing short of suicide. The USSR of 1941 was in the middle of massive restructuring following the Fall of France (basically, Stalin thought slow trench warfare was the way to go until the German Blitzkrieg proved him wrong, then he tried to rush a complete overhaul, which left the USSR with its pants down when Barbarossa started). In 1939, it would be using outdated tactics, but it would be a lot more organized and even somewhat actually prepared.
Plus, it would be rasputitsa season; Poland fell by mid-September, by which point all Russian roads would basically have turned to mud. As 1941 proved, nothing's worse for an attack than to fight knee-deep in mud and sludge.
So yeah, invading the USSR right after Poland would have shortened WW2 by... 5 years. German gets smashed as it gets bogged down in a war it's even worse prepared for than OTL, potentially Soviets on the Rhine, definitely in control of Warsaw and Berlin.