Using a proxyconf.pac file leads to slow opening of https-pages - why?

Hi experts.

I really need someone with a little knowledge about proxy configuration files, because this problem starts to go to my nerves.
First of all and very important: there is nothing broken on windows here. It's on all computers in our company and the problem seems to originate in the way some browsers interprete proxy config files.

We run both IE9 and chrome (latest) on vista business SP2 x64/x86. Both browsers experience the same problem while Firefox does not: when connecting to https websites (example: https://www.paypal.com), the address in IE9 bar goes green immediately to indicate a secure connection, but until the page is displayed, it takes more than a minute.
-this does happen with IE9 and chrome (latest)
-this does happen with all https sites I tested
-this does NOT happen with firefox
-this does NOT happen in any browser if I don't use a proxyconf.pac file but set the proxy to be: ourproxy with ourport.
-this does NOT happen if we keep browsing that site (once opened, it goes fast)

The proxyconf.pac has been examined and we even shrunk it to two lines for a test:
----
function FindProxyForURL(url, host) {
    return "PROXY OurProxy'sIP:OurPort"
}
----
but nevertheless it stays slow... You can see, the proxyconf file does indeed do nothing special and anyone here that has a proxy to test with can at once use it for a test.

What could that be?