ScotRail's class 318 units. They were originally designed for the fast Ayr to Glasgow services, and they have no problem running long sections at 90mph.
In 2011, the new class 380s took over all the Ayrshire Coast services, so the 318s have started being used on slower local services on the Argyle Line. They run the local services pretty well, but they're not getting the speed they were designed for
Semi-interesting point: In the mid 2000s, the class 334 fleet were being used for a lot of the Ayr to Glasgow services. In 2008, word spread on train forums that the 334s weren't doing great on the Ayr to Glasgow line, and ScotRail wanted to move them somewhere else. I never heard it confirmed by anyone official, but I noticed a lot more Ayr services were class 318s after that.