In certain scenarios, when there are a lot of slowly responding hosts it is possible that even 1000 pollers are not enough. In one example examined a proxy with 1000 pollers is doing ~1000 NVPS While it's not overloaded in any other way, but pollers are 100% busy. Investigations showed that there are a lot of hosts that do answer, but do it very slowly.
Very often seeing output on pollers like:
Collected 4 values in 10 seconds.
Currently the only solution is to deploy more proxies.