This was added in 2.1 for whatever reason. All instances of F_WriteText are already capped to the framerate properly by other means (including the text prompts added by 2.2); so this code being present just makes timing differences between different netgame clients / other inaccuracies cause the text speed to slow down more than intended, eventually causing desync in netgame cutscenes.
Resolves #1033 (closed) (I made said issue since I almost gave up on investigating the cause, until I decided to compare both Final Demo and 2.1's codebases. Oops.)