Squid available from outside

It has been pointed out to me that my Squid proxy server can be
accessed from outside our intranet. I do not want to go to user
authentication (yet!), could I do something like

acl US src 'our ip address range'

http_access deny !US

If so would this slow down Squid much?
Is Squid likely to slow noticeably from a large number of ACL's?
If so what would that large number be?

BTW I have sorted out the time restrictions I was trying to do with
CRON (they worked) but the acl time rules are much smoother,
once you work out the sequence and the rules about ANDing and
ORing!

--
Simon Bryan                    sbryan@olmc.nsw.edu.au
Information Technology Manager sbryan@mpx.com.au
OLMC Parramatta