Holding speeds
 

Was just wondering recently why the maximum holding speeds increase with altitude 230/240/265kts etc?

At the lower altitudes where there would be more traffic, the holding pattern protected airspace is more confined. The slower speeds (200 KIAS in the USA below 6000') would give smaller radius of turns.

At the higher altitudes, the larger radius (and longer legs) is less of a problem, so faster speeds are allowed.