Now you have to have already created a fixed timing track to be able to alter it in the XML. That's what I bet they didn't realize. Now this isn't changing the minimum sequence duration. That's in a different spot.
I added a timing track (right click, Add Timing), gave it a name but nothing else.
Saved the sequence. Closed the sequence.
Opened the XML in Notepad+++ and searched for
Element Type. Clicked next until it found my new timing track
Added
fixed="50" to the end of the line but before the
>Before:
<Element type="timing" name="fixed 50msec">
<EffectLayer/>
After:
<Element type="timing" name="fixed 50msec" fixed="50">
<EffectLayer/>
Saved the file. Reopened in xLights and it now has fixed timing of 50msec (20 ticks per sec)
I will say this - having a fixed grid that matches your sequence timing makes alignment of effect starts and stops easier to see, but you really have to zoom in.