I'm running MariaDB 10.2.31 on Ubuntu 18.4.4 LTS.
On a regular basis I encounter the following conundrum - especially when starting out in the morning, that is when my DEV environment has been idle for the night - but also during the day from time to time.

I have a table (this applies to other tables as well) with approx. 15.000 rows and (amongst others) an index on a VARCHAR column containing on average 5 to 10 characters.
Notably, most columns including this one are `GENERATED ALWAYS AS (JSON_EXTRACT(....)) STORED` since 99% of my data comes from a REST API as JSON-encoded strings (and conveniently I simply store those in one column and extract everything else).

When running a query on that column `WHERE colname LIKE 'text%'` I find query-result durations of i.e. 0.006 seconds. Nice. When I have my query `EXPLAIN`ed, I can see that the index is being used.
However, as I have mentioned, when I start out in the morning, this takes way longer (14 seconds this morning). I know about the query cache and I tried this with query cache turned off (both via `SET GLOBAL query_cache_type=OFF` and `RESET QUERY CACHE`). In this case I get consistent times of approx. 0.3 seconds - as expected.

So, what would you recommend I should look into? Is my DB sleeping? Is there such a thing?