Speed of copying data from PostgreSQL primary node to read-only nodes

I have created a PostgreSQL database cluster, that consists of one primary+standby database and three read-only databases (each read-only database for a specific region).

During development I’ve faced with the next issue. When I write a row into database and then immediately (but after a promise is resolved of course) read the table, then I’ve got dataset without those new row (in most cases, but sometimes the dataset contains all expected rows).

But if I put some delay (600ms, for example), then the dataset contains contains all expected rows for all cases.

My question is: Does such behavior relate to some latency that is appeared because of copying data from primary node to all read-only nodes? If so, then how many time is needed for such copying? Or maybe, if the time is floating and depends on many factors, then how is it possible to measure this latency for my case?

Thanks for the help!