TL;DR
Make a third buffer for orders outside roboport reach for robot tasks.What ?
I am making a fairly large base and was expanding via laser grids, and other large blueprints. I noticed a slowdown where it looked like the game did nothing. Based on that I believe that your current robotorder system works with 2 buffers, a 600 (in my case) active orders that tries to be filled that slowly time out, with a max of new orders in the buffer, and a larger buffer with everything else.Now if you place a blueprint outside roboport reach it still goes in the larger buffer, but only X orders are checked per cycle, so if you have huge orders outside reach (landfill, pavement, or large laser grids/solar panel groups without roboports) a lot of those tasks can not be filled, but the buffer still uses its precious checks if they are available. This in term means that you get huge slowdowns where nothing is done as all that is checked is inaccesible stuff.
My suggestion is that everything works as now, but inaccessable things are moved to a third buffer when found (still cost 1 check, but no rechecks and initial check can be done on placement), and whenever a new roboport is put online its area is checked against this larger buffer, and if anything is found they are moved back into the secondary buffer.
A possible downside would be bases which turns roboports off and on a lot (power issues), but beside that the large scale constructions outside the roboports should be more achieveable.