Another aspect of the speed of the technique is the nage *may be* setting the speed instead of allowing the uke to control their meeting as a means to gain firmer control overall.
Our sensei often says that the nage sets the pace....no matter how fast uke comes at him, he gets sucked into the technique with balance just out of reach, either before him (faster) or just behind where it is recoverable (slower).
At times it can look like uke charges full blown, into nages solo slow motion tai-chi form.
Doing a technique exquisitely slow to reveal a point he is stressing is also MUCH harder than doing it with flow and momentum. Lots of benefits.