My generators do require more calculation than many types of engines - especially the nearly-calculation-free "set the output to this (maybe with a simple condition) and then just sit there" type, but I am dubious about how much processing is really going on. These entities run almost no code - much less than something like a mob - and seeing as the game can easily handle large farms with hundreds (or even thousands) of mobs, I do not forsee that being an issue. As for the reactor code itself, even the fuel rods (the most complex) use a simple algorithm:Ofcourse. But flexibility often come with a high tradeoff; resource consumption. I'm assuming you're going to test this extensively because sofar your power generators require a LOT more calculations than say a bunch of boilers for example. And it's just the 'heat' generator part, you also need some form of turbines (but that's hardly an issue, animations are mostly a client-side strain). Don't forget you want to be in modpack together with a lot of other mods that already take their share of CPU power to run. I would hate to see all your hard efforts be worthless because your mod got a reputation of being a resource hog.
Regarding what you're doing; it's a lot more complex than simple calculations. Counting is easy for CPU's, branching sucks for CPU's and you're doing a lot of branching. Misprediction leads to pipeline flushes. Pipeline flushes leads to the dark side.
Test chance -> True:
Destroy neutron entity; Spawn 3 more, and set their directions to a random horizontal from ForgeDirection() (2+nextInt(4)); add heat;
Additionally, I have looked at the code from other mods' engines, and they are not as light as people seem to assume; BuildCraft engines in particular have a LOT of per-tick code.
Really, the only lag I am noticing seems to be related to (unrequested) block and lighting updates, and that is the vanilla code doing something I do not fully understand. Come to think of it, that seems to be the main overhead in RotaryCraft and ExpandedRedstone as well. (Well, RC also has somewhat heavy rendering load, from complex models).