To call the train service from Truro/Plymouth high speed is pushing it a bit, and that's before part of it falls into the sea without hope of replacement in a few years time.
Damn right, over 4 hours for the quickest service to London, with stops at every village on the way till you get past Exeter. From experience I think Groundhog was a bit lucky doing Falmouth - Heathrow in just over 4 hours, but its still way faster than rail to Reading and change or Paddington and back. Only time I ever used Newquay was when Ryanair were flying to stanstead and I could catch a connection to Paderborn - as a Cornwall - London service it was a dead loss when you factored in check in times, etc, so even the slow train is preferable!