Yes, rendering was on, see screenshot. Switching it off made PM6 much more stable (had no crash so far) and browsing thumbnails is smooth. Browsing previews is smooth too, but very slow: 2+ seconds to jump to the next one. This is on Mojave.
Back on High Sierra now, PM6 is very stable and very fast, except with rendering switched on: PM6 stalled (with beach ball) after 28 images while browsing previews. Had to force quit PM6 after about 10 minutes. Have an Apple crash log if you want it.
UPDATE: I installed the Adobe DNG Converter to render the NEF's instead of Photoshop CC 2019. To my surprise, that made a difference. Browsing previews is slower and speed is irregular, but DNG Converter seems to be more stable; no crashes so far. Switching RAW rendering off is clearly the best option, both in speed and stability.
In numbers, browsing 177 NEF files in Preview (28%):
High Sierra / Photoshop: 20 seconds, crashes possible
High Sierra / DNG Converter: 31 seconds
High Sierra, no rendering: 16 seconds
Mojave / Photoshop 18 seconds, irregular speed, crashes possible
Mojave / DNG Converter: 33 seconds, irregular speed
Mojave, no rendering: 15 seconds
My preliminary conclusion:
Having RAW rendering switched on causes trouble, on Mojave and to a lesser degree on High Sierra. With RAW rendering switched off browsing previews is very fast on both High Sierra and Mojave.
If you really want RAW rendering, use Adobe DNG Converter, not Photoshop.