I don't see how. More RAM isn't going to make the content of that video file any easier for your CPU to decode.
If your issue occurs only when recording or streaming, instead of just running OBS with the overlay on, it might help to see a logfile of a completed output session, but from your post I understand the performance issue occurs immediately when enabling the overlay, and since hardware decoding is not set to on for those media sources, it uses your CPU for decoding. If that's causing your CPU to hit 100% then the issue is that media file.