Which version of Veeam MP do you have in your environment? 150 or even 200 VMs should not be an issue for our collectors at all (in case you have 6.x and later version). I would recommend to open a support ticket and upload collector logs and Operation Manager event log from these servers to our tech support team, so that they can investigate the issue.
Thank you very much for your interest in Veeam MP product, sorry that our product doesn't perform well, but maybe something else is happening on these servers and I'm sure we can fix it. Our collector should be able to handle up to 1K VMs on one collector.