You don’t need to force the halves together quickly to start a chain reaction, but you do need to put them together fast to create a bomb. If it’s not fast enough you will get a “Fizzle” [0] where some chain reaction is occurring but not over a small enough timespan to make a bomb or to stop the material from disintegrating itself. A similar slow chain reaction process is used to control energy release in nuclear power plants.

[0] - https://en.m.wikipedia.org/wiki/Nuclear_chain_reaction#Prede...