[exim] Limit concurrent deliveries

I am running exim4 on a server with very limited resources. and using a local
pipe transport for spam filtering. Is there any way to limit the number of
processes run when doing local deliveries. At present a transport process is
run for every queued message whenever the queue is scanned. I have tried
queue_run_max = 1
smtp_accept_queue = 2
but these do not limit the concurrent transports run.
Thanks,
Peter