I already posted this:
In the last trailer Rowlet has 47 HP at level 16 and in a different screenshot it has 50 at level 15 (consider that Tepig max HP at level 15 are 49). Now assuming it has something like 67 base HP (if 50 is the max it can reach at level 16) in pokemon history when a starter has a stat over 60 in its base form, that stat is never below 100 in its last stage. Now, assuming that Decidueye HP will be somthing like 105-115, looking at the speed chart of all pokemon, excluding Legends, Mega and Slaking (which all have a lot of base stats), there is not a single pokemon with base HP over 100 and a base speed over 95 (only Darmaritan has 105 base Hp and 95 base speed actually, when nobody else have more then 100 HP and more then 90 speed). So. in my opinion the best speed Decidueye can reach is 90.
The only reason we had to think that Litten was really fast was because it is faster then rowlett (which we thought was a really fast pokemon too). But if Rowlet will have 80-90 base speed, I'm convinced that Incineroar will not have more then 100, like gen 1 starters, cause seriously that thing looks bulky and I can't see it being faster then Infernape.