In general, faster airflow is better.
Sticker slots have an effect too. ¾” stickers with 1000 ft/min is about the same as 5/8” stickers with 1200 ft/min (same CFM).
Cost is a consideration. The cost goes up with the cube of the fan speed. By going to 80% fan speed at the end of the cycle you use about (08.x0.8x0.8=0.51) 51% as much electricity. It is about 73% power use at 90% fan speed.
If you still have a decent depression on the exiting air side, consider reducing fan speed to save money late in the schedule. What’s decent? You should probably reduce fan speed if you have a 40F depression entering and 30F exiting. You should not reduce fan speed if the exiting depression is less than half the entering.
While we know that reducing fan speed has minimal effect on how fast a single board dries, reducing fan speed does increase TDAL and extend the drying time on the leaving side. Therefore, slowing the airflow during the last 25% of the schedule will extend the drying time somewhat. This is cost trade off.
Use a reduced fan speed during equalizing and definitely during conditioning.
Reducing fan speed early in the cycle will increase wets in the middle of the load and noticeably extend drying time. Some kilns are set to use a reduced fan speed when the kiln is cold to avoid pulling too many amps so don’t mess with this unless you are watching the motor draw.