A lot of enterprises, operators and organizations block or rate-limit UDP traffic outside of port 53 (used for DNS) since it has in recent days mostly been abused for attacks. In particular, some of the existing UDP protocols and popular server implementations for them have been vulnerable for amplification attacks where one attacker can make a huge amount of outgoing traffic to target innocent victims.
QUIC has built-in mitigation against amplification attacks by requiring that the initial packet must be at least 1200 bytes and by restriction in the protocol that says that a server must not send more than three times the size of the request in response without receiving a packet from the client in response.
This seems to be the truth, at least today in 2018. We can of course not tell how this will develop and how much of this is simply the result of UDP transfer performance not having been in developers' focus for many years.
For most clients, this "slowness" is probably never even noticeable.
Similar to the "UDP is slow" remark above, this is partly because the TCP and TLS usage of the world has had a longer time to mature, improve and get hardware assistance.
There are reasons to expect this to improve over time. The question is how much this extra CPU usage will hurt deployers.
No it is not. Google brought the initial spec to the IETF after having proved, on a large Internet-wide scale, that deploying this style of protocol over UDP actually works and performs well.
Since then, individuals from a large number of companies and organizations have worked in the vendor-neutral organization IETF to put together a standard transport protocol out of it. In that work, Google employees have of course been participating, but so have employees from a large number of other companies that are interested in furthering the state of transport protocols on the Internet, including Mozilla, Fastly, Cloudflare, Akamai, Microsoft, Facebook and Apple.
That is not really a critique but an opinion. Maybe it is, and maybe it is too little of an improvement so close in time since HTTP/2 was shipped.
HTTP/3 is likely to perform much better in packet loss-ridden networks, it offers faster handshakes so it will improve latency both as perceived and actual. But is that enough of benefits to motivate people to deploy HTTP/3 support on their servers and for their services? Time and future performance measurements will surely tell!