I hate to say, but your attack speed also increases with Base Attack Bonus, so for a fair test you'd need a lvl 15 with Rapid Shot and a lvl 15 without it.
Assuming you're full Artificer (+4 conjured bolts) your BAB went from 10 to 11, and per
MrCow's testing as part of Vanshilar's Attack Speed post your BAB increases your attack speed by roughly 2.9% between 10 and 11.
And Repeaters have weird attack speed anyway due to 3 shots per reload animation.
Admittedly that's smaller than the 7.5% gains shown, but it should be noted if you're looking for valid test results. Maybe try it again after retraining Rapid Shot away with a free swap from Fred?