Not sure if this is software / hardware related, or both. I've read dozens of "suspend" problems. Seems to be endless # of reasons for suspend issues & appears many don't find a fix or reason.

This is an older Toshiba laptop, Intel NIC. Running LM 19.3 XFCE.
Normal booting is pretty fast ~ 60 sec or less. I have to manually time how long the monitor resumes after all other disk R/W have stopped. But there are lots of processes that don't involve disks.

I can only look at logs, the time it appears all "waking" processes have finished and see if that differs much from when the monitor wakes.
I'm actually more interested in turning off disk drives after long inactivity

Using an external monitor as the primary. The laptop display is completely disabled in Settings > Display. At one pt, I thought though the laptop monitor was turned off, it wasn't unchecked as "primary" might cause problems (while external monitor was also marked "primary)." But changing that didn't affect external monitor resume times.

If I manually send it to suspend, at least sometimes, resume of everything & monitor is 3 - 5 sec. That may? be because in manual suspend, it doesn't shut everything down right away.
But if it suspends based on time set in XFCE Power Manager, the monitor take 60 sec or more longer, after other processes seem finished & logs seem to show it's fully unsuspended.

There are a few "couldn't find / do / set... " type entries, using journalctl, but the times shown between them & next entry is insignificant. I can't find where the big time, or dozens of temporarily stalled processes take the amount of time = to the monitor's resume delay.

I also haven't found in logs any entries like "display resumed" or "monitor awake." I don't know what that event would be called, but it's not intuitive.

On my older Toshiba laptop that I normally use with an external monitor, I append kernel boot parameters to disable the laptop display and enable the external display.
The names of display outputs on your laptop can be found from the output of 'ls /sys/class/drm'.

Thanks for the info, allend,
1st, I'm no expert on using external monitors on laptops (I only "maintain" it for another). I'm not at the laptop, just now.
However, it would appear in the XFCE "Power Manager" utility, the laptop display is both marked "inactive" (or UNchecked "active"), and NOT marked as Primary display, anymore.

I still really need some input about what to look for in say, using systemctl plus what options & commands, to see what's going on during the time from when I 1st hit a key to wake laptop from suspend, until the display finally appears. I'm less sure than before if using an external monitor has anything to do with the relatively long time for the display to resume after "ending" suspend by hitting a key. I don't think it matters what key (I haven't seen a difference in resume time based on hitting diff keys).

From outward appearances, since the changes in Power Mgr (unchecked laptop mon. as active or "primary"), I haven't seen / heard a peep from the native display. Changing the display settings in Power Mgr didn't change the system / display resume time, AFAICT.
None of this means other things aren't still affecting suspend & resume behavior. Like suspend / resume control from the kernel or other system files, etc. (just guessing).
I see a few "Error" or "Warning" entries in Systemctl, but the total time listed in systemctl, from hitting a key to where it shows "suspend exited," doesn't seem to match the CLOCK time it takes from hitting a key till the display resumes.

Linux kernels do have ACPIs that control suspend & many other things, but I've not found a doc with specifics. For now, I'm clueless what a given kernel version does with suspend, w/ or w/o settings entered in something like XFCE Power Mgr.

(side note for others using LM XFCE - or other DE, in Power Mgr's "Display" tab, there were several instances of settings for both the laptop display & external monitor. EVEN though I didn't (intentionally) create all those "profiles").

Next, I may not try appending kernel parameters, yet. For future cases, which method did you use (& exactly how), to append those boot parameters?
In grub's conf file? If so, how did the boot parameters look, after you added it?

The problem with suspend and resume issues is that they can be hardware specific. As you do not seem to need to be switching displays, I am suggesting you try running with only the external display enabled, to see if the problem can be made to go away.

A web search for 'grub append kernel boot parameters' provides plenty of sources of information on temporarily and permanently adding kernel boot parameters.

I have a couple laptops with broken displays I picked up from a friend. His brother told me he got frustrated while using the Samsung and flung it across the room smashing it on the wall.
It's firmware settings are set to boot from the drive only... apparently, since I can't boot a CD/DVD or USB. I want to put a couple Linux on it but there is no way to access the UEFI settings. The attached monitor be it though VGA or HDMI does not get fired up till Windows log in shows up. I can only run Windows on this thing since I'm guessing Secure Boot is also still enabled.

I took it apart to the point I can unplug the broken display which rumor has it should automatically make the attached monitor default, that has no effect. I do have it set as the only display in Windows since the lit up broken display is rather unpleasant to have to look at. I've tried every trick in the Wild Wild West (WWW) and cannot get any sort of display through external ports to work like the factory supplied monitor.

The other one, a fancy HP with Win7 Home Premium, similar issue. A couple times it didn't boot properly so I slapped Super Grub2 Hybrid in the optical drive and just wait till it's spun up, hit enter, wait a few seconds, hit down arrow key once and hit enter and the Windows 7 starts appearing on the attached monitor.

I have to do it blind, I'm guessing the only Linux I'll be able to install on that one would be the ones that install from live mode, no text mode or console based installers. Haven't tried to fire up a Linux DVD on that one yet. But damn does it have a wicked DVD burner with that lite scribe or whatever it's got, so... that's it's purpose.

Anyway, neither one can use an external monitor as a replacement for the original. The external will not yield the same results.

Coincidence:

I just compiled 5.15.1 kernel, as I was going through the differences between it and the 5.14.15 kernel config I noticed this new feature similar to:
I'm pretty sure the allow EFI was already available with prior kernel, but not VGA.