Install and rebuild were quite uneventful - nothing to whine over ... Maybe of the contrary - some 2-3 bugs with apps in 14.1 had resolved themselves meanwhile. And it helps to have previous install-list of apps, deps and shit.
But when attempting to install (previously used in 14.1 ) binary Nvidia driver 331.49, I had sudden 'unknown libEGL.so' problem, and no install. So I went for the latest beta - 334.21, which installed with no problems.
... Well, week before or so also smxi in my Debian Sid had got the same driver installed (after Xorg update, as the 'current nvidia'). And I had started to experience some lagging with mouse and keyboard. Sorta, occasional 1-2 seconds before slider moved, or mouse click registered or typed letter appeared. Bloody annoying, that! But, frankly, I thought that the culprit is some other critter of myriad of Sids' upgrades - and that it goes away with next bunch.
Now, after I installed the same driver to Slackware - and at once started to experience lags, I sure reconsidered. As an afterthought - it was most probably some kind of not-syncing between this Nvidia driver and Compton compositor.
Anyway, I didn't want to part with Compton, so I downgraded the Nvidia driver - and successfully, despite:
1. Slackware - previous installers' libEGL crap and no install.
I went to runlevel 3, did separate
sh NVIDIA-Linux-x86_64-334.21.run --uninstall
without letting Nvidia to restore previous xorg.conf (no idea if that matters), then
slackpkg search mesa && slackpkg reinstall mesa-9.1.7-x86_64-1
to be sure I don't have leftovers from Nvidia (as I heard, Nvidia overwrites some of mesas' files). Then:sh NVIDIA-Linux-x86_64-319.49.run
with 32bit support and 'yes' to overwrite xorg.conf.
Success, no whining from installer, AND - no more lags!
2. Sid. Not with smxi help this time, but manually:
telinit 1
sh NVIDIA-Linux-x86_64-334.21.run --uninstall
That finished with installers' ominous whine about leftover symlinks. Whatever... I decided also not to bother with mesa and press on and see what happens:
sh NVIDIA-Linux-x86_64-331.49.run
'Yes, do install' - to installers' worry about 'maybe runlevel 1 is not good...'
With 32bit support, 'Yes' to overwrite conf.
Success, and no more lags!
Simpleminded conclusion: If you experience lags (especially after pointless upgrade to latest/greatest Nvidia driver), try downgrading a bit. Beta really was Beta, in my case.