#1 I had installed the release candidate of the Linux kernel version 6.15 and was normally running on 6.14. There were hibernation timing issues with 6.15 and I opened a thread for that: but it was not addressed as the kernel was not in release yet.
#2 During the update 6.14 was uninstalled from my machine. I installed 6.14-rt to replace it and ran time tests using a stopwatch.
Hibernation with Linux614-rt were much like the times for Linux614 (18.14 seconds)
Hibernation with Linux615 are longer (105.06 seconds) 579% increase!
I have Plasma on Wayland, but the times hibernating from Openbox and X.org are very close to the same.
My questions:
#1 Am I the only one with this condition?
#2 Is if this is a known issue and if anyone is addressing this either here or at the Kernel team?
Do you actually need the latest mainline kernel(s) for your hardware? I’d suggest sticking with a long-term-support (LTS) e.g. linux612 if that works for you.
Also, this might possibly be relevant:
Cheers! 
So, when the release notice says that Kernel 6.15 is supported in stable it only KINDA means it?
I’ll have to find the thread but it was mentioned it was still missing something in the Stable branch. This might have been sorted out by now. 
I’ll need to check the other machine as am on my Mint netbook right now.
EDIT: I just needed to look a few posts up on this one, didn’t I. 
6.15 is not an LTS version
The suggestion is/was to use an LTS - one that works and will keep being supported for quite some time,
and fall back on it in case a newer one displays problems for you.
Unless you have a specific reason to go for something different.
It does work, no? 
Just not as good as expected for you.
and: perhaps it is a firmware issue and things will change
Will drop back to linux612 (or Linux614-rt) and keep Linux615 for my testing only pending firmware package update.
Thank you!