I searched for variance and apparently nobody mentioned this before: They talk of Mean request time: 306ms Median request time: 46ms Which indicates a very high variance, so don't take for granted that an x50 increase of performance would result from intelligent routing. The problem is that the fast tasks suffer from being queued after the slow tasks, so each such fast task takes an extra latency. If the variance is lower, the random routing will be favorable at some point as the delay of getting the task from the router queue to the dyno is not zero neither. In the case of no variance, "intelligent routing" would always add that delay as soon as all dynos are at their limit. Before that, the router would simply keep a list of idle dynos and send work there without delay.

Sure if you never hit 100% load, intelligent routing is cheap and comes at no delay. Imagine 40ms jobs getting all dynos to 100% load. Now the dynos would be idle for the duration of the ping that it takes to report being idle. let that be 4ms. That is 10% less throughput than with items queuing up on the dyno.

The router being the bottleneck would therefore justify to make it stateless and give the dynos a chance to use these last 10% of processing power as well, ultimately increasing the throughput by 10%. Sure, a serious project would not run its servers at 120% load hoping to eventually get back to 100% within time, so all this being said I would always favor intelligent routing to get responsive servers, add dynos in rush hours and only opt for dyno-queuing for stuff that may come with a delay (scientific number crunching, …)