Multipliers don’t stack, they’re additive (that’s the only thing that makes sense, and how we handle the rare instances where they come up). I don’t know about all the other weird stuff being added in (I don’t play 5th ed, but that’s a lot of movement and action-type specific stuff that’s probably not meant to stack, either). You’ll still get a ridiculous number, assuming the DM allows all of those bonuses to stack on each other (a lot of magical bonuses have limitations regarding stacking, mostly AC-based ones, but they’re not exclusive)…but not quite THAT much.
Found out today that a level 18 tabaxi monk, under the effects of a haste spell, wearing boots of speed, and using feline agility, and then using both action and haste action to dash, combined with step of the wind bonus action dash, can technically move 1920 feet per round. That’s 218 miles per hour. I’m getting nauseous just thinking about it.
5e handles stacking differently – namely, everything stacks unless it it changes your base. So something that is ‘your movement speed increases by 10’ stacks just fine with similar effects; something that says ‘your base movement speed is now 40’ doesn’t.
The ‘double a double is a triple’ thing still holds true though, and I feel like a level 18 Tabaxi monk/barbarian/fighter going that fast for one round isn’t as big of a deal as a level 9 wizard who can teleport anyway. :P
