slow growing table

HI!

I have a table that I use for about a month. As the month progresses,
COPYs performed to this table get much much slower than they were at
the beginning, for the same number of rows (about 100,000 and
growing).

I'm essentially doing a delete for a given day, then a COPY as a big
transaction. This is done about 12 times a day.

When the table is new it's very fast, towards the end of the month
it's taking almost 10 times longer, yet I'm deleting and COPYing in
the same amount of data. Other operations on this table slow down,
too, that were fast before using the same criteria.

I do a VACUUM ANALYZE after each delete / COPY process, I tried
experimenting with CLUSTER but saw no real difference.

this is psql 7.45 on Linux server, dedicated for this purpose. About 5
indexes, no FKs on this table.

happy to provide any other info might need, suggestions appreciated

all my best,
Jone