Yup, it's the neighbour function being the problem. And of course updating even more boids, but that just scales linearly, but its noticeable when decreasing the neighbour radius which gives each boid less other boids to adjust to.

My neighbourhood implementation is the naive n^2 loop, so it's probably trivial to make it better. Quadtrees, or even just simple caching / zoning would probably allow me to dish out many more boids.

It also doesn't help that in order to be easy as a workshop, it's written in a way that's easy to implement, but slow to execute.