In my opinion Rest and GRPC are different technology and they don't be a rival. The GRPC can be preferred on streaming progress on the other hand this is not acceptable for rest :)
If you want to still compare grpc and rest speed, I can share a link for you.
I think you can get the performance problem answer at the given link below
https://www.yonego.com/nl/why-milliseconds-matter/#gref
Spoiler: the light load progress Rest can be faster than grpc but if you compare them under high load, GRPC can win this race :)
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…