The AgendaX Update Service has a negligible impact on the Exchange Servers. When it scans the Exchange mailbox calendars, it does so by scanning each configured mailbox individually one at a time. You can therefore compare the bandwidth and processor usage of the AgendaX Update Service with a (very fast working) user that opens each mailbox one at a time and reads every calendar entry that takes place in the interval specified by the Administrator (days past, days ahead switches in AgendaXCfg.exe). Scanning a calendar takes anywhere between a few milliseconds to a few seconds, depending mostly on the speed of the Exchange Servers and network throughput between the AgendaX Server and the Exchange Servers.
The above applies to a single AgendaX Update Service configured to scan all the mailboxes into 1 database.
You can also install several AgendaX Update Services working in parallel on a single machine, each scanning different mailboxes either into the same database (to cut down time) or into several databases (to handle access permissions or visibility requirements, e.g. maybe some groups should not see other groups’ calendars).