basically if the url changes very frequently, there is a lot of (async) sql queries made to the places database. Even though it does not affect CPU usage that much, it creates a lot of async requests to the database, which queue up and take a lot of time to get compelte and subsequently increasing the File IO (not necessarily the DISK IO) too much.
Refer bug 814942 for full details.
basically if the url changes very frequently, there is a lot of (async) sql queries made to the places database. Even though it does not affect CPU usage that much, it creates a lot of async requests to the database, which queue up and take a lot of time to get compelte and subsequently increasing the File IO (not necessarily the DISK IO) too much. Refer bug 814942 for full details.