[blueskin_]

I haven't seen this in ~1k services, but I guess it probably depends on the spec of the monitoring system to some degree, and I realise that 1k+ hosts is likely a different story. If you're using passive checks in any high-rate capacity, you should be using NSCA or increasing the frequency they are read in anyway. This is also another problem Icinga handles better - while I say Nagios for convenience's sake, my comments here refer to Icinga (and to Nagios XI, which is comparable but stupidly expensive).