I have practiced TKD for a while and I am wondering at what point are kicks too slow. Let me give an example below: My height is around 176 cm at the moment and I timed both my roundhouse kicks and side kicks to the waist level, done 75~90 cm from the target (a B.O.B the same height as me, in this case). They average around 0.32 seconds from the beginning of movement to the point of impact before warm-up and 0.24 seconds after warm-up. Does this seem a bit slow in real life combat? Or how fast should kicks usually be?
From a TKD specific position, as it's an Olympic sport, we can look at the studies which have been carried out about it from a sport's performance perspective.
The primary test of TKD specific ability is the Frequency Speed of Kick Test, which is carried out over a single 10 second bout, and then repeated 10 second efforts. This was developed by the Spanish National Team (and has been restudied on others), and is essentially the most kicks you can complete in the 10 second periods.
National level competitors managed about 20 kicks in the single bout, dropping to 16 by the 5th repeated bout.
Regional competitors managed about 16 or 17 kicks in the individual 10 second bout.
From working as an S&C coach with a national level youth athlete, she achieved 18 kicks in the individual 10 second bout, and only dropped down to 16 by the end of the repeated efforts.
This is with bandal chagi (excuse Korean spelling) or a crescent kick, but similar numbers of kicks can be achieved with mawashigeri/roundhouse kick (sorry, I don't know the Korean) - this is however based off of my anecdotal experience as an S&C coach and karate instructor, not any studies done.
This would give us a kicking speed for most National level athletes of around 1 kick every 0.55s from floor to floor - assuming that due to the rebound from a heavy bag the return is slightly faster than the first portion of the kick your times of 0.25-0.32s for the first portion are definitely quick enough from a sporting perspective.
Due to the shorter path I would expect straight kicks (particularly front kicks) to be quicker, but can't locate any studies on this at this point.
As kicks generally - and high kicks particularly - are less tactically sound for self-defence situations, I wouldn't worry over much about the raw speed of a technique for this situation.