How make my architecture scalable to handle linear process
Since you suspect the disk is the limiting factor, upgrading to SSD sounds like your best option if you are focused on hardware.  More RAM might also work, if it would allow to cache most of the slow table/indices.  (How big is map_rto and its most-used index)?

If you would rather try tuning the hardware you already have, or making sure that that is really the issue before buy hardware, then you should identify the slow queries from your stored procedures, and follow this advice for them.

Particularly useful would be turn on track_io_timing and post the EXPLAIN (ANALYZE, BUFFERS) for the slow queries.

Finally, is the code that calls these two functions single-threaded?  If so, you could try making it parallel, with different database connections for each thread.