Slow as in it has high latency issues but once it's transfering (from a single source) it's fast.
Basically, that's inherent in the technology as when it gets transfered throught the 3 proxies it will take longer and so the speed may be more akin to a 56k modem in the time it takes to find the page and set up the transfers. Once it's transfering it'll perform at pretty much full speed that is unless content on the page is linked from various other sites in which case it'll be very slow as you'll have that latency issues with every single piece of content that is off-site - then it will be completely performing like a 56k modem.
In short, it's fine for relaxed browsing where time isn't a precious. However, if time is critical and you do mind the extra few seconds transfering from page to page (say 4 seconds), then it won't be suitable. Equally, if you're having to deal with alot of content linked from different sites then it'll also be unsuitable unless you're willing to endure some truely painful loading times.
It has it's place and is a good technology but I don't bother using it.