I reckon travelling between mission signal sources in that system takes a long time with so many gravity wells. It's especially a PITA when they decide to stick as close to a gas giant as possible. In case of San Yax, how far out do mission signal sources normally spread?
In general, no more than a 5,000Ls radius, so at-most 10,000Ls, in my experience. Sometimes they can spawn close to GGs, yes, but there's ways around that, and most massacre/assassination USS spawn in deep space for me, so there's virtually no speed-up time, usually.
Also, just for the record, I spent 12 minutes stacking across 4 board generations (Sveit 22:39, 22:41, Tripitaka 22:48, 22:51). Despite 5 factions being out of commission due to Election, Civil War and Expansion, I got the following missions (kills/reward):
Massacres: 30/41, 9/7, 9/7, 24/26,16/13
Assassinations: 6 totalling 15 million
Factoring in how they get stacked, that's a total of 109m for 40 kills.. but will be closer to 125m after bounty claims as well. Those 40 kills will easily get knocked over in =~ 40-50 minutes. But if those other 5 factions weren't out of action, would have easily been 150-200m.
But that's also if you just "do those missions" which is fairly ineffective... generally I'll head back when I run out of assassinations (I don't actually seek these out, I just wait for them to jump out at the massacre USS and include them in things) or if I'm down to two or three stacked missions left. Then I just head back, reload/rearm, grab additional massacres/assassinations and continue things.