It's quite hard to describe it as "slave labour", since there isn't really a concept that matches the wartime production of Soviet Russia, in capitalist Allied countries.
The working conditions were terrible, but the Allied production lines weren't exactly too labour-friendly either. Although the harshness of working conditions were undeniably much worse in the USSR during their bad years in WW2, this can't be adequately described with the term "slave labour". Nazi Germans with Jews in their workcamps were "slave labour". USSR was not.
It's a weird mix of terror from above + grand patriotism + obligatory reaction + fatalism. The punishments and reprimandations of not meeting production quotas were severe and inhuman, and yet, as terrible it was, the Russian people accepted it as their fate - the war wasn't just about the soldiers at the frontlines. The war was at the production lines also; either you succeed in meeting the quotas, or the country dies.
While we tend to think that the gruesome reality of Soviet Russia would make its people loathe the system, quite contrarily, the people of Russia were generally content and very proud of their country, and often fanatically patriotic, at least in WW2.
The warring years were, ironically, the most "democratic" years of Russia since 1905~1917, as there were no exceptions or previliges to anyone. One's status may be high or low in society, he may be a party memeber or a "nomencalatura" or not.. there was no exceptions. If someone couldn't contribute to the war, whether he is a soldier or a general, a common labourer or a high ranking Commisar, they couldn't survive.
Everybody had to do something. The Russians accepted that, and they did what they had to do to win a war, which, should they ever lose, mean the end of everything. Hitler and co. made it pretty clear that if they won, they were gonna literally wipe everything 'Russian' off from the face of the Earth.
If the workers were just purely driven to produce by terror, Soviet Russia would have never made it through 1942.