I resisted jailbreaking for a long time. I decided to give it a try since it's easy to revert. When I first jailbroke I didn't install anything. Used it for a week like that and I couldn't tell the difference from my stock phone (makes sense.)
I did not install Winterboard at first because of everything I heard about it. What I really wanted was clippy or hClipboard. I installed hClipboard and still didn't really notice a difference. Maybe slight, but it could have been in my head. Certainly not a bother.
To keep things simple and continue testing, I uninstalled hClipboard and installed Winterboard, and downloaded the "badge black" theme which does change the look of quite a few things. I only checked the "badge black", "badge black UI" and "user wallpaper" in Winterboard. I really don't think the phone slowed down much, if at all, and I was quite happy with the new look.
I reinstalled hClipboard and additionally installed the 5 icon dock and LockDockBar. Finally I am now seeing some slowdown on the phone. If I restart it each night before I charge it, it remains reasonable throughout the day and I really like having the new look and new features.
Then I deleted LockDockBar and replaced it with StatusNotifier, and also installed the 5-row qwerty keyboard. That was the limit for me - the phone seemed really sluggish. I went back to LockDockBar (I prefer it because it shows you the number of txts and emails) and got rid of the 5-row keyboard (I am so used to the stock one that the 5-row drove me nuts) and now all is well.
My conclusion is that the things that really seem to slow down the phone are things that use MobileSubstrate. Right now I'm down to LockDockBar, Winterboard, and 5-icon dock, and the phone seems snappy.
Wow, that post was too long.