This page list possible problems with Collector installation, and suggests solutions for them.
One particular scenario might overload the Collector: If you collectively restart your IIS instances, you should not have more than 50 Web Server Agents (depending on configuration) connected to one Collector. With normal operations, you may be able to connect more Web Server Agents to the same Collector.
Slow application startup
If you experience high application startup time, chances are that the network latency between the Agents and the Collector they connect to is too high and the number of round-trips add up. Move the Collector closer to its Agents, avoiding firewalls in between if possible, or at least use a very fast one.
Collector class cache and inheritance map sizes
Do not underestimate the impact of Collector class cache and inheritance map sizes. Despite recurrent automatic cleanup (by default every week), they can get quite big quickly. You can set cleanup recurrence with (maximum age in milliseconds; below: one day):
For details see KB-484 - How to fix a class cache explosion.