Thank you. The Bridgewater-Brunel server is set to 16,384 tiles per step, so each city should be taking 1.5-2 minutes when players are connected, thus taking between 460.5 and 614 minutes (7.6 - 10.2 hours) to complete the route search for all 307 cities.
With the rate multiplied by 4 and multiplied again by 6 (for multi-threading) when there are no clients connected, this should take between 19.18 and 25.5 minutes.
Having logged onto the server a little earlier, I see that the routes have, in fact, refreshed, presumably whilst nobody was connected. Memory usage seems fairly high, however, at ~8Gb, presumably because there are now far more private car routes in the world.
This suggests that performance is workable provided that there are significant periods (i.e. at least half an hour) during which players are offline each day, which does indeed seem to be the case.
One possible way of significantly reducing the computational load of this system if necessary would be not to calculate separate routes to industries and attractions inside cities, although this would not be ideal in gameplay terms, as doing this is specifically intended to allow for load balancing of different road routes through cities. For this reason, I do not want to do this unless necessary, and it appears as though it is probably not necessary, although I am minded to start the next game with fewer towns. I should note that the number of towns in this game has increased considerably as a result of player built towns, which has thus increased significantly both the computational load and memory requirement of this system. Consideration will need to be given as to what, if anything, to do about this in future server games.