Time Trial / Time Trialling / TT

I saw some riders grimly hanging onto their tribar position while grinding up climbs at very low speed; surely it should be understood that the modern time trial position is only beneficial when the rider is travelling at a reasonable speed.

Do you know for sure what that speed is? Many is the time I've seen somebody ahead doing much the same speed as me up a slope, and when they get off the tribars they suddenly come back towards me.