Constant PARALLEL_MIN_WALKS 

pub const PARALLEL_MIN_WALKS: usize = 3;

Number of walks from which Simulator::new builds them on the rayon pool rather than serially (#860). Measured on an Apple M5 Max (18 cores) with 30-step geometric Brownian walks: one or two walks build no faster on the pool than serially, three already build faster. Longer walks only move the crossover lower.