We have around 400 users that are setup to use Cloud Web Security via the PAC file. We've had numerous complaints of slowness while using the PAC. If the PAC is not used, browsing is tremendously faster.
Browsing seems to be adversely affected as the day goes on and is normal, as far as my experience, during the morning hours.
If we select "Authenticate only in these cases" rather than "Authenticate users on first access" under the Access Control tab, browsing seems to be much better, however our Websense reports show [Unspecified] for all traffic instead of actual usernames.