It wasn't much of a war, so analyzing the performance is difficult
When the Red Army had attacked Poland from the east on the 17th September 1939, it had used 465,000 troops and 485 tanks. Against them, there were 12,000 border defense troops and, a bit more to the west 200,000 mostly poorly armed conscripts, that were planned to be used to patch the loses in the west.
In the evening of the 17th, Marshall Rydz-Smigły, the Commander-in-chief of the Polish forces made an unclear proclamation:
„Sowiety wkroczyły. Nakazuję ogólne wycofanie na Rumunię i Węgry najkrótszymi drogami. Z bolszewikami nie walczyć, chyba w razie natarcia z ich strony albo próby rozbrojenia oddziałów. Zadanie Warszawy i miast, które miały się bronić przed Niemcami – bez zmian. Miasta, do których podejdą bolszewicy, powinny z nimi pertraktować w sprawie wyjścia garnizonów do Węgier lub Rumunii”.
The Soviets have invaded. I order general retreat towards Romania and Hungary. Do not fight with the Bolsheviks, unless they attack you first or they try to disarm your troops. Warsaw and other cities defending from the Germans - no change to orders. Cities approached by the Bolsheviks should negotiate to allow the retreat of troops towards Romania and Hungary 1
(note, by "Bolsheviks" he meant of course the Red Army).
As you can see, the order was to "if possible, retreat without fighting", so most forces were doing exactly that: either avoiding fighting or surrendering quickly. Still, some commanders decided to ignore the order and fight, even managing to achieve a few small victories routing and destroying the Red Army forces in Szack, or Parczew.
So in summary, whenever the Red Army met resistance, the results were close to a draw, as both sides were ending up with heavy loses. If not for the fatal order, eastern Poland could stand and fight for quite a while, but in the end the Russian victory was inevitable.