Postgres cursor taking 2 hrs to update the table

Hi,

I have the 10 application processes(each process having 12 threads and each of have updating 8 million rows in the shared memory) which are reading the data from single postgres database. We are using database cursors in C++ to fetch the data and updating the shared memory. There is the timer attached, where all the processor threads going to fetch the data.

The issue is that sometimes it is fetching all the data in few seconds and sometimes it is taking 2 hrs.

What could be the issue. Can you please guide me to resolve the issue?

Regards
Tarkeshwar