An ISO when written to DVD media is very slow; as that’s not the intended media for installation; these days the intended media is USB-thumb-drive, or loading the ISO direct off disk/ssd.
Yes it takes ages on DVD; the last ISO I burned to DVD media I think was in the groovy cycle (or hirsute) because of changes in the boot process & the thumb-drives weren’t booting at that time, I burnt it to DVD to see if it burnt there - results were identical to thumb-drive just far slower with the various dailies (just taking much longer).
All of Ubuntu ISOs & flavors of Ubuntu (such as Lubuntu) are created in the same manner, and it varies per release - intentionally so it’s the same process for all architectures (be it amd64, arm64, s390x, ppc64el, armh7 etc). Ubuntu 20.04 LTS media was the last before these changes were implemented (where amd64 was different to other architectures on focal and prior releases).
Ubuntu have no intention of changing this, or speeding up DVD media as it’s only a used by a handful of people. Note: I’m repeating here my understanding of what I’ve gleaned from discussions upstream over various issues, some of which I’ll alluded to; slowness was not ideal, but as it still worked to install it wasn’t a showstopper
Your issue may not be the DVD media itself, some boxes take >10 mins from thumb-drive due to issues related to firmware, this will likely add extra time if DVD media is used too, but that issue is not serious because the media still boots; and is only an issue with the ISO itself, so once the system is installed all is perfect.
No new media will be created until Lubuntu 22.04.1 LTS media is created in a few months; unless a series flaw is discovered which will force it. To get issues like what you describe fixed, they need to be discovered in alpha or beta stages & fixed before release & be deemed show stoppers; release has already occurred. The slow boot was seen as a documentation issue primarily as it affects systems only prior to installation (and an estimated fraction of 1% of users).
firefox was strongly tested; it had it’s on discord room during the beta cycle (as Lubuntu did too) and some issues were noted (and bugs filed), but all but a few have been resolved with the remaining ones being worked on by Mozilla. The snap daemon issue maybe related to media (were there any squashfs errors?, did media verification complete successfully? etc) or related to DVD media itself, but if it was DVD related, a bug report would need to be filed & issue is unlikely to be fixed prior to Lubuntu 20.04.1 LTS media, especially if the install itself works (ie. fix is likely to be deemed to be a documentation issue I suspect given it’s DVD installers only - but that’s my opinion only!)
Your testing is now too late, as we’ve released our Lubuntu 22.04 LTS product already, but thank you for testing, but try and perform the testing next time before release so we can act on it.
Please note: The opinions I’ve expressed here are my own. Much of the decisions as they relate to ISO media would be done by Ubuntu teams upstream with more understanding than my own. I’m largely a QA (Quality Assurance) tester in those processes.