I can't help but wonder why you need such a new kernel for such an old machine (no judgment... I have much older ones in service)? and did you build it yourself using "make oldconfig" first?

Well, I suppose I don't. But the laptop isn't my 'main machine'. I use it when I need to take it somewhere, but mostly for testing. I nearly always install or upgrade something on there first, so I can try it out before upgrading the PC I depend on. For example, I just recently put LibreOffice 6.2.6 on it, while my main PC is running 6.1.6 but I'll upgrade that soon.

The laptop also boots Xubuntu and Windows XP, but that's another story.

Also, I use the Slackware-provided generic-smp kernel. I haven't needed to build my own kernel since ... I can't remember. But I did it when we had floppy disk distributions. Building a recent kernel on a Pentium M with 2GB would probably be painful.

The 2GB are more than enough for a 1 job make and the Pentium M would do it in a few hours 2-3 (I might be wrong).
Before going for your own compilation, I'd suggest to compare the .config files between 4.4.186 and 4.4.189, maybe you can catch something interesting there.
The kernel config for 4.4.189 is available in:
https://mirrors.slackware.com/slackw...p-noarch-1.txz
The one for 4.4.186 maybe you have it on your system ... it's gone from the Slackware mirrors.

Some extra check - are you sure your HDD is sane on that old computer? Could you just copy (read) the 4.4.189 files into a folder, watch if you experience some delays - check with dmesg for any HDD related messages. Using smartctl for a report might also be useful.

Disk is OK. Not as old as the laptop, checked frequently with smartctl, and not logging scary messages.

Slackware kernel config for 4.4.186 is identical to 4.4.189.

Just for fun, I built 4.4.189 from source on the laptop. You are correct, 2GB is just about enough, although it did use a tiny bit of swap. Mostly it takes a long time because single-core and slow processor. You were also right about the time: a little more than 3 hours.

I used the Slackware config but with one change: CONFIG_X86_VERBOSE_BOOTUP changed from 'y' to not set. It just turns off those messages that appear so slowly, during that 10 minute interval. The goal is just to see if the slowdown is the messages themselves, or something else.

And with those messages disabled, it boots fast as before. Which means the slow-down is actually from outputting those messages. Which is very strange.

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Happy to hear you made it work.

Maybe you could engage with the kernel devs and report this issue.

Accidental help is what I do best!

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Followup: The problem does not occur with the just-released Slackware-14.2 update to kernel-4.4.199. The laptop boots at normal speed. (I never upgraded it to 4.4.190, so I don't know exactly which version fixed the issue.)