The "speed_multiplier" attribute of the component "minecraft:boostable" has seemingly no effect on what the sprint speed is when boosted. It's like the value is being overwritten internally.
1) In the pig entity, search for 'minecraft:boostable'.
2) Change the value to 10.0 for "speed_multiplier"
3) In game, summon, saddle, and ride the pig. Holding the 'carrot on a stick', use it while riding to boost the pig.
4) Compare the sprint speed of the pig now, vs when it had the "speed_multiplier" of 2.0.
Expected: Pig sprints at 10x walking speed, per "speed_multiplier" adjustment.
Actual: Pig still sprints at 2x walking speed.
MCPE-48129, which was unresolved at the time.