2.0.7: Less then 40 sec delay between /api/v2/write insert and dashboard graphing

__Proposal:__
Reduce the time between data insert and graphing the values.

__Current behavior:__
At 1 second insert intervals (using http://127.0.0.1:8086/api/v2/write), and a dashboard graph at "Past 5 min" (subsamping to 10 second data point), the delay (missing/empty measurement values in the graph) is about 40 seconds. Example: at 14:16:00 the last point visible is 14:15:20.

__Desired behavior:__
Reduce time between insert and graphing.

__Alternatives considered:__
Going back to not so stable release 2.0.4.
Choosing a shorter interval (Past 1 min), wishing to see 1 second measurements. #21721 feature is not available.

__Use case:__
Why is this important (helps with prioritizing requests)?
To see (near) live data.

Start by reproducing the delay with 1-second inserts through the /api/v2/write endpoint and a dashboard graph set to Past 5 min, comparing the latest inserted timestamp with the latest visible point. Trace the ingestion-to-dashboard path to identify where the roughly 40-second gap occurs; done means the graph shows newly inserted values with a substantially reduced delay.

Written by the indexing model from the issue text.