If the subtransaction cache is overflowed in some of the transactions then it will affect all the concurrent queries as they need to access the SLRU for checking the visibility of each tuple. But currently there is no way to identify whether in any backend subtransaction is overflowed or what is the current active subtransaction count. Attached patch adds subtransaction count and subtransaction overflow status in pg_stat_activity. I have implemented this because of the recent complain about the same[1]
