One more question.
It seems I have been wrong to say there is no pattern to the slowdown.
Giving it another look, the slowdown seems to correlate with the number of workers.
Working with 20 processes (in addition to main process) would give slow down every 20 iterations (not exactly but close to, with some variance).
Moving to 10 processes will cause shorter intervals between slow downs.
Am I missing some other route cause for my slow downs? related to initialization of workers?