You’re not being specific with times, but slow reads from a DVD are not new.
During the groovy cycle there was a stage when USB thumb-drives wouldn’t boot on certain old HP devices, so I wrote a ISO to DVD to test to see if DVD would boot (given USB wouldn’t; 2020-07-17) which reminded me how much slower the media is.
Media is almost always written to thumb-drive these days; almost no-one will use a DVD which is a much slower device.
The currently ISO will validate itself (thus all the DVD will be read - this takes awhile!) though if you’ve enough RAM, the cached data will make it fast once completed.
(Note: there are a small number of devices using a firmware that is very slow to boot live media, be it on DVD or thumb-drive; on those devices ~11 mins can be expected for a thumb-drive & I have no idea how long it would be using DVD media. Bug reports have work-around to make an ISO boot faster if you’re using hardware with this firmware issue; but it wasn’t tested with DVD media; and once installed the system is fine - it only impacts the live or installation media; you didn’t give times so this may not be your issue)