Just having cydia on your phone will make it slower. Try this as a test: open text messages; close it. (see how long it takes) now, open cydia; close it; open text messages; (notice how much longer it took?) I just did that right now using my gf's compared to my stock phone.
She has NOTHING installed from cydia/installer; yet mine turned on faster, turned off faster, connected to safari faster, and opened text messages and e-mail faster. What was the point of JBing you phone if you don't install any apps? Your post leaves many holes.
So sorry, I must have missed that day in school about computer architecture

But until I see a JB iphone that runs as fast as my stock phone, i'll stop talking. You on the otherhand can keep continuing with your useless banter, kid. P.S. Take a class on grammar... until then... don't talk
Yea, I realize my grammar isn't as good as my skills with computers, but I also don't go making posts on Grammar forums about usages of comas and semicolons.
I have done that with Cydia but I don't see any differences. I'll explain a little to you about what programs do when they run. Most CPU's have a Chache (really fast ram) and then memory (ram) and then there is normal storage space. Now when you run a program, Cydia for example, it loads it to the Ram and may have to free up spaces in the ram for it to fit. It then caches blocks of memory so it can have faster access to them, this happens often and gets cleared often. Now, if you exit Cydia, you then have to re load SMS and then the phone can start cacheing it. It should have the same effect if you open any large program and try to open your texting program. PS: if iPhone architecture works differently, then I'll admit that I am basing this on the way most computers work. I think that the ARM architecture is the same though.
If you use terminal or SSH you can do a command called 'top' which shows all running aps and their memory usage. When I close Cydia my usage is where it was before I opened it.
Also, you must have missed the post where I explained all that do without installing the apps that DO slow down one's phone. I'm not someone who just likes to say they "hacked" their phone by running a program they got off the internet. No bragging in that. I have use and if you don't, then don't bother with it.
@Spaz I wasn't trying to imply that you guys were saying that just Jailbreaking slows you down. More towards people like this Katuls guy. I'm not even going to make any more personal comments though, because I can only see it escalating to flaming.
If you want to know how to edit some of the themes manually to keep your phone fast I can point you in the direction of some treads. It requires the use of basic linux commands, but it isn't that hard to learn. I taught myself how to do a lot of Linux work by reading forums and I'd be glad to return the favor. All images on your phone are stored in specific folders, so if you can find the icons or the image files you can replace them with any ones you want. As long as the image is the same size it's not going to have any performance impact. What I do is download a theme, SSH to it and copy the images from it and manually do my replacing. The other option, no linux knowledge needed is to use an SFTP client. Cyberduck is one. you can also just Google "SFTP free Windows" or "SFTP free Mac" and find one.
To know where to put the images is a little trickier, I don't know where they go, per say, but I just Google what I'm doing. Like "ssh battery image iphone" and then I can find out how. the /Applications and the ~/Applications folders have icons for your Stock and App Store apps accordingly.
I should make a tread on ways to Jailbreak and stay fast. Haha
EDIT: forgot to address Categories. Yea, it's so slow, but only when it's loading up a category. it doesn't slow down the phone in general. It's annoying though to wait a minute for my category to open, that's why I only put my least used apps in them. I wish there was a faster version. Oh well.