It has been several years since I played with this, but there is a
delicate balance of population size, number of genes, mutation rate,
cross-over rate, and how quickly the population fixates on an answer.
If your population is too small, then it fixates, and you are dependent
on mutation only. If your population is LARGE, then it takes forever to
converge on a solution.
When I used DEAP before, I played around a little to find a nice
balance. It has been long enough, I do not remember how many genes and
population size, seemed to work well. I should add that my evaluation
function took 1 to 10 hours to run for each individual, so it was worth
the time it took in tuning.