Is it normal that using OBS with the Skype NDI integration is unusable on mid-2015 Macbook Pro?
I'm assuming Skype is decoding and re-encoding the video stream at that point.  If so, then yes, that's probably entirely normal.  You have two MPEG encoders running at the same time.  My 2017 MBP (i7) struggles to do that with Zoom.