Should the T and L phase of a well tuned ETL report process take the same amount of time regardless of data size? I have two different production systems where the total time for an ETL (Extract, Transform, Load) reporting process (Power BI Report Server) takes around 47 to 60 minutes whether the extract phase pulls data for 15 minutes, 1 hour, 12 hours, or 24 hours. Yes, there are vastly different amounts of rows for those ranges.
Is this normal?
I had expected that the total time for ETL when there is NO data would be .... well... a small amount of total time.  But I must confess that I know nothing about the internals of the T and L phases within a report server.
Can someone shed light on whether the T and L phases are typically fixed in duration (up to a point)? I suspect that this process has not been examined or tuned.

UPDATE
In regards to the T&amp;L phases, I have a theory but no proof.
In the extract phase for these two systems, the total time for the queries remains about the same. 6 minutes regardless of how much data is being pulled. Why? Because a handful of the queries use clustered index scans. I've documented this problem in other questions.
My theory/fear for the T&amp;L phases - is that something similar is happening; a clustered index scan such that it doesn't matter (to a point) how much data is being processed.
But as I mentioned, I have no knowledge (yet) of the inner workings of a Power BI report server's Transform and Load phase. I am making simple observations and curious why the total time does not change when no data is pulled.
There's nothing specifically quantifiable about the abstract concepts of Transform and Load, only their concrete implementations are measurable. To be able to comment on your case, we'd need to know specifically what your Transformation and Loading processes are actually doing. Obviously some Transformations can take multitudes longer than others.
But generally speaking, the amount of data being processed definitely should affect the overall runtime of an ETL process. If there's a significant difference in the amount of data between a 24 hour timeframe vs a 15 minute timeframe, yet your ETL process is executing at roughly the same average runtime for both cases, then something is definitely suspect, and that's not normal.
Even if an index scan is occuring in both cases, if there's a significant difference in the amount of data, the total runtime should certainly reflect it. The runtime of an index scan is linear (generally speaking) based on the number of rows in the index.