I think I might have discovered the source of frustration here. I ran into the same issue after replacing the display on my wife's late 2014 MacBook 13". The display replacement went great thanks to the iFixit detailed instructions, but when I reassembled and turned it on I got the high fan RPM symptom and throttled CPU symptom.
Googling the problem showed me that I wasn't the only person this had happened to, however, it didn't seem that there was a definite fix, except letting the Apple techs have at it.
I remembered, during reassembly, having a bit of difficulty getting the camera cable inserted, and remembered thinking that I was having to use far too much force to get the connector to seat. I had inserted the connector in the orientation in which it laid naturally on the logic board assuming that it was correct.
I also recalled seeing the gold pins of the camera connector during disassembly of the faulty display facing me when I removed the cable. So I flipped the cable over and it slid easily back into the connector and seated securely without force. Voila!
Everything is working perfectly now. Anyone attempting to replace the display on a late 2014 MacBook 13" should be aware of the fact that the flat camera cable might not be oriented the correct way for insertion. Make sure the gold contacts are facing you when inserting!