I think it really depends on what you're after and the micro location where you spend your day and night. Hvar and it's city is definitely not laid back and slow. It is just very expensive, busy and loud. Korcula is fine, but I would suggest something like Vis island or for someone appreciating very small islands: Susak or Ilovik. Being on all of those places many times in my life, I think it mostly depends who you're with and where your apartment is. Every island can be very diverse in itself as well. Let me explain: if you're placed in a town usually it will be very loud on weekends and nights due to open air concerts/performances, and sometimes even during the weekdays. So if you desire something where you'll get a nice sleep this can actually be hard to come by because every cafee is open during the whole night each day entertaining a few drunks, blasting music, and not letting hundreds of people sleep with open windows (nights are hot as well). Actually it was much more bearable with covid restrictions in place because the cafees weren't allowed to play music after IIRC 10PM. A lot of islands also have no police or very minimal presence, so there is a lot of elements of lawlessness and locals making their own "rules".

I could write pages of this stuff, but let me write more positive things. The best way to enjoy the islands in Croatia is actually by boat, rented or otherwise (they are not expensive). There are beautiful quiet coves where you can spend your days and nights, only going for resupply in towns. You anchor in a quiet cove, and in the evening you take your tender boat to the town if you want to walk around it. Next day you sail to another island and repeat. That experience is actually awesome.

If a boat is out of the question, being in some apartment is an option, I would just advise not to be in the town center or near it, or anywhere where some very local beach is. Mostly there will be a caffee with blaring music and nobody likes to sleep with earplugs. Also this is and advice for any island, and true, a lot of those islands are quite different than each other.