What is the cost of a tx?

In the current system that i am working on the feature in question
requires between 1 and 4 requests (individual tx) to a read replica for
extra data (assume orm usage where entity b must be fetched if entity a
doesn’t exist ant so on). At scale this ends up being between 250k (best
case) to 1m (worst case) requests per minute. The individual tx setup uses
"read committed” tx mode. I am considering to move those 4 requests into
single transaction to reduce the amount of transactions in general, but
without behavior change there would still be up to 4 individual queries
during that transaction, so database doesn’t really have full information
about what I want from it. Is the logic sound to perform the change or (at
my scale) the overhead is negligible where it doesn’t really matter that
it’s 1 larger transaction that takes 4 time units compared to 4 smaller
transactions that take 1 time unit?

I suspect that using repeatable read would push the change into single tx
direction since I wouldn’t be snapshotting the database per query.

I am also aware that I could just measure the change but sadly I do not
want to test in production, and this question is more about learning about
other’s experiences.

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// Mantas