Running out of memory is a RAM problem
The title is in reference to the cause of the problem which in this case running out of memory is only a symptom. The fact that the game is playable until a certain amount of time shows the problem lies not in limitations of ram but an inefficient system built into the engine.
Its no so much what they save, but how they save it.
So if the save file is inefficiently stored but also loaded into memory for fast saving shouldn't the problems be fixed if
it was removed? A test was done where a larger save file that had slowdown was then loaded into a new game file
and the slowdown persisted. Yet the same new file loaded first after a restart had no issues.