I had noticed that filter performance in 1.4 had seemed sluggish compared to 1.3 and earlier. It appears that a 250ms delay (verified by inspecting the source JS) has been introduced. As described here:

http://api.jquerymobile.com/filterable/

The filterable widget reacts to the change signal by reading the value of the input after a short delay and iterating...

Can anyone shed any light on why this might be?

It gives the appearance of being far less responsive because of the delay.