Tweaks Logon Changer doesn't slow down your boot. All it does is replace the default image file with a scaled down / size limited image of your choice. I've used it on pretty much all my Win7 systems (including my system here at work). There's a warning that pops up about the image being too big and that it will scale it down for you even. So no - it shouldn't slow down any boot times because either way - it's loading a simple image. It doesn't run on boot every time or anything like that. You could replace the image yourself manually if you wanted to even. This is just a nice GUI to do it for you.
I can't say one way or the other about Stardock's logon changer since I haven't ever used it.