XnView Software

Where did the detailed .avif settings go?
In XnViewMp 1.6.1 There is only a simple percentage slider left: For comparison, there used to be many more fine-grained options in XnViewMP <= 1.5.5 I could set min and max for colours and speperate min and max for the alpha channel.

Now we use libheif to save AVIF format, and different min/max are not available

That new library actually seems to to use multi-threading now and has therefore become much faster.
To benchmark this, I compared conversion times between XnConvert 1.98 vs 1.99

I chose to convert a 15.85 MB PNG file (4000x4000 pixels) to AVIF
Hardware: AMD 5700X 8-core/16-threads
14 threads have been assigned to XnConvert.

XnConvert 1.98: 4000x4000 PNG -> 1050x1050 AVIF
9% CPU, 39 seconds XnConvert 1.98: 4000x4000 PNG -> 4000x4000 AVIF
9% CPU, 4 minutes 44 seconds (sic!)
XnConvert 1.99: 4000x4000 PNG -> 1050x1050 AVIF
30% CPU, 12 seconds XnConvert 1.99: 4000x4000 PNG -> 4000x4000 AVIF
35% CPU, 1 minute 4 seconds MUCH better. Could still use some more threads, but it's significantly faster...
Resize went from 39 seconds to 12 seconds.
Full conversion went from 4min 44sec to 1min 4sec.
1.99 is basically 4 times (sic!) faster than 1.98.

The latest features are in XnViewMP available only.

So I prefer XnViewMP for conversion jobs.
In most cases it is as fast or faster than Xnconvert,
because it has the newest file plugins and better multi core support.