1.55 seconds.
How did I get this number?
This half-year 38 people have been faster than 3:35, which shows a significant increase in depth compared to the last 10 years.
I compiled a list of best performances of each first half-year of the last 10 years for comparison (more than 750 performances in total).
Here's what I got:
The average time of a 38th runner of those past 10 years is 3:36.52.
So this year elite 1500m runners have been ~ 1.5 seconds faster in average than runners from the last 10 years.
0.1s per 100m, 0.4s per lap.
Seems plausible, open for discussion