I rolled my own kernel a few cycles ago and encountered a Wifi issue and video issue with kernel 5.
NOTE: I ended up going back to standard Slackware-current kernels after this... (fun, to try new kernels, but only when you have time for testing)

Now that our beloved Slackware-current is running 5.4.x see below if you have flickering screen and an Intel i915.

Good news both are resolved one in kernel (the wifi issue)

(The wifi was fixed by disabling a new feature) disable TX-AMSDU on older NICs [more here]
https://bugzilla.kernel.org/show_bug.cgi?id=203577
- The patch was placed in kernel code so no action required.

Issue 2: This video chip, for me, maybe others too from intel
Intel 00:02.0 VGA compatible controller: Intel Corporation HD Graphics 620 (rev 02)
The other https://wiki.archlinux.org/index.php/Intel_graphicswhere I found this but this was critical for me so I thought I'd share.

If you experience random (often under load) issues where the screen flickers and blacks out for seconds at a time.

add this to /boot/efi/EFI/Slackware elilo.conf or /etc/lilo.conf
maybe it will help as it has for me thank goodness
It really had me annoyed....

append="i915.enable_psr=0" is for disable power saving settings, right?

Exactly