logical replication initial copy and WAL generation

Hello Forum,
I know it sounds weird but we have faced it a couple of times already and
have no idea.
We're trying to establish logical replication from RDS(10.9) to RDS(12.5).
We're adding tables to the publication one by one.
During the initial copy for some tables we experience huge WAL generation
on the source db.
From 0.5mb/s it jumps to 15MB/s.
This happens when a copy just started or in the middle or at the end.
WAL generation causes service statements to wait on WALWriteLock causing
high response time.

Any idea how the initial copy can cause WAL generation ?!

Thank you very much,

Rado