I wanted to better understand how the arm speed attribute correlates to bowling speed and also understand why Medium pacers seem to be a bit too slow in this game, so I've taken note of how fast certain bowlers in this game bowl in relation to arm speed (on a 50-99 scale), and these are the results for a standard straight seam delivery with good timing on jump input and release point;
Fast bowler, maximum (99) arm speed: 151.2 km/h
Fast bowler, 82 arm speed: 148.1 km/h
Fast-Medium bowler, maximum (99) arm speed: 140.4 km/h
Fast-Medium bowler, 85 arm speed: 137.9 km/h
Fast-Medium bowler, 75 arm speed: 136.0 km/h
Medium bowler, maximum (99) arm speed: 126.0 km/h
Medium bowler, 82 arm speed: 120.4 km/h
So arm speed makes a subtle but noticeable difference to the speeds of Fast and Fast-Medium bowlers, and the drop-off in pace appears to be similar for both bowling styles, with a 10.8 km/h difference between a Fast bowler and a Fast-Medium bowler if arm speed is the same on both, and presumably a 9-9.5 km/h difference between a bowler with maximum arm speed and one with minimum.
Medium pace, on the other hand, is quick enough when arm speed is at or close to maximum, but the pace falls away very quickly, with what appears to be an over 16 km/h difference between the fastest and slowest Medium pacers. What myself and others can take away from this is that right now, it's almost a necessity for a Medium pacer who's known to be a solid part-time or even first change bowler in real life to either have an arm speed attribute that's at or close to maximum, or otherwise be classified as a Fast-Medium with minimum arm speed.
Perhaps the correlation between arm speed and bowling speed for Medium pacers could be tweaked in patch 3...