I am assuming xlights only recognizes up to 20 since that is all available in nutcracker and you can't type a number... however if you wanted to try you could go into the xml file and change the 20 to a higher number and see if it would work...
example: E1_SLIDER_Speed=20
change speed to say 30? or 50? or something and see if it works?
Again I don't think it will but I don't know.... Maybe Sean or Matt will be able to tell you for sure.
**Note** obviously you would have to do this for every arch every time you wanted it to be faster, so I would say only test a few before having to edit the entire xml file.....
If it does work you can do a CTR+F for the specific chase (because slider is part of all effects) and then replace them all