Inconsistency of computational time

[tanmayGIT]

Hi, I am performing an experiment to measure STUMP's computational time, which uses STOMP_Opt at it's origin.
I have a 1D time series (even tried with 4 different time series) of 100k length and am increasing the subsequence length by 2000 in each iteration to see the change in computational time.

Ideally, the computational time should decrease but it is kind of fluctuating. Another point is why always the computational time for the very first iteration is very high ?
I suspect that it may happen due to the use of threads and the thread synchronization issues. But whether there are any concise details about the reason for it. I need to write in my article where the reviewer has asked to compare the performance by using the STUMPY library.

Here is one example:

The sub-sequence length 2000 and the needed time is 19.032682 ??
The sub-sequence length 4000 and the needed time is 10.091411
The sub-sequence length 6000 and the needed time is 10.302999
The sub-sequence length 8000 and the needed time is 10.007654
The sub-sequence length 10000 and the needed time is 9.512854
The sub-sequence length 12000 and the needed time is 20.557093
The sub-sequence length 14000 and the needed time is 17.757711
The sub-sequence length 16000 and the needed time is 17.917705
The sub-sequence length 18000 and the needed time is 17.200630
The sub-sequence length 20000 and the needed time is 15.299617
The sub-sequence length 22000 and the needed time is 15.026888
The sub-sequence length 24000 and the needed time is 14.469026
The sub-sequence length 26000 and the needed time is 13.799839
The sub-sequence length 28000 and the needed time is 12.855113

Thanks in advance for your help and more insights.