Comment by mzs

Oh that's it, thanks! In fact it it returns a 200 not 301 then:

  % curl -gsSw'%{time_total}\n' -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.6 Safari/605.1.15' https://t.co/DzIiCFp7Ti
  <head><noscript><META http-equiv="refresh" content="0;URL=https://www.threads.net/@chaco_mmm_room"></noscript><title>https://www.threads.net/@chaco_mmm_room</title></head><script>window.opener = null; location.replace("https:\/\/www.threads.net\/@chaco_mmm_room")</script>4.690000
  % curl -gsSIw'%{time_total}\n' -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.6 Safari/605.1.15' https://t.co/DzIiCFp7Ti
  HTTP/2 200 
  ...
  content-length: 272
  ...
  x-response-time: 4524
  ...
  
  4.660211

The delay is not there for nyti.ms (anymore) but once you use the Safari UA it's handled as 200 response:

  % curl -gsSIw'foo %{time_total}\n' -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.6 Safari/605.1.15' https://t.co/4fs609qwWt https://t.co/iigzas6QBx | grep '^\(HTTP/\)\|\(location: \)\|\(foo \)'
  HTTP/2 200 
  foo 0.126043
  HTTP/2 200 
  foo 0.037255

It really does seem that twitter is adding a 4.5s delay to some sites from web browsers. Could be malicious, could be rot...