Tonguing velocity

I played trumpet for 7 years in middle school+high school and just started picking it up again after a 6 year break. I'm surprised at the range I managed to retain (I can still hit the high C!) and even the tone quality (it's not as good as it used to be but it's not horribad). However it seems my tonguing speed has dramatically decreased. I can still sort of double-tongue but it sounds a little sloppy.

Which brings me to my questions: how exactly do you improve your tonguing speed? Even when I was actively playing in high school I always felt that my tongue moved too slowly (iirc I can't do 16th notes at 120 bpm) so I think this is an area I've always struggled in. Also, what is an acceptable cutoff to switch to double tonguing?