Tor will slow down browsing much worse than ads.
In general, yes. But there are things you can do to mitigate that. A lot of the tweaks that I've made to T4Fx originally were to wring more speed out of the browser when I was using Tor.
Now, the Tor Bundle won't run on a PowerPC Mac so you have to use Vidalia in conjunction with Torbutton. You can tell Vidalia via it's torrc configuration file to only use certain nodes and certain sockets, to switch identities within a certain time frame and so on. Some Tor nodes are slower than others, some just blow all together. It's just based on what the node operator set up.
For me, the major downside of Tor is that Google is constantly changing as your IP changes. Note that you can't determine which IP address you get to use nor can you control your specific Tor exit node. All you can control is what range of nodes to use.
One last thing to add. You don't need to use a Mozilla based browser to use Tor. Just fire up Vidalia and make sure your proxy in System Preferences>Network is set to the correct port for SOCKS. Any browser you use will then be running on Tor.
Of course none of this is as secure as using the Tor Bundle or Tails, but if you want that kind of security you aren't using a PowerPC Mac.
Oh! You want to see slow? Start Vidalia and let it connect to the Tor network. Then start and configure the JonDoAnym network using the JAP application to use localhost only. Set your apps to use a specific port that you set in the JAP application.
Result? Your applications communicate to the JAP application. JAP talks to localhost which goes out to the Tor network. Essentially you are using Tor as a tunnel to the JonDoAnym network. Tor anonymizes and encrypts your data and when it comes out of the Tor exit node it goes straight to the JoDoAnym network and gets anonymized and encrypted again. You can be in Phoenix, while your exit node is in Sweden, then hits the JoDoAnym network and finally exits the other end in Germany or somewhere.
All of that though DOES equal SLOW!