Fetching rows degrading in a high resulset query

Oracle DB version 19

When I run a query returning some milions of rows fetching them is getting more and more time per row

I run the query by sqlplus locally at database instance and I spool output in a file

After execution time seems to be finished (excutions = 1 in v$sqlstats ) I start to observe fetching

At beginning of fetching phase I can observe around 20000 rows fetched per minute

Than fetching rate slow down and after two hours I can observe less than 10000 rows fetched per minute

Do you know how I can improve fetching time ?

I tried tuning arraysize and rowprefetch sqlplus parameter, but nothing seems to change a lot