Search not flowing as proportionally on an activity

with regard to the bar showing the amount done it is as I would think it should.

It starts and travels ahead before it gets about across and then pauses a long period of time before the running is finished.

Why doesn't it go evenly?

So as 30 seconds passes it looks like this



by 35 seconds it looks like this



for 40 seconds having passed it looks like this



in 1 minute 20 seconds it looks like this



on 2 minutes 40 seconds it looks like this