TL;DR: Nothing that's inherent to QUIC itself, it's just that current QUIC implementations are CPU-bound because hardware GRO support has not yet matured in commodity NICs.
But throughput was never the compelling aspect of QUIC in the first place. It was always the reduced latency. A 1-RTT handshake including key/cert exchange is nothing to scoff at, and the 2-RTT request/response cycle that HTTP/3-over-QUIC offers means that I can load a blog page from a rinky-dink server on the other side of the world in < 500 ms. Look ma, no CDN!
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