The controller should have sped up the first one, unless it was a simple sequence of two.
"For sequencing/separation, increase to 300k"
IMHO there should be a rule that above ten jets descend at 295 +/- 15kts unless advise (before descent point) or instructed otherwise.
The cost index is BS as referred to earlier it only provides system-wide savings if you're the only aircraft in the sky, anyhoo it's all moot with low oil prices.