Simply rooting won't really speed the phone up, but rooting it and installing a custom image or rooting it and then removing some of the built in applications could speed it up.
Here is a recent thread by another user showing scores (the scores are a benchmark test where it kind of ranks the phone after a series of different tests)
http://androidforums.com/sony-ericsson-xperia-x10/257499-ran-quadrant-standard.html
What you want to know before you do anything is what version you currently have and what you want to try and achieve.
If you don't want to void warranty, don't root.
If you have Android version 1.6 installed, you might want to upgrade to 2.1 and give it a go.
If you want to go that extra step and really make your phone fly and remove some of the Sony applications that you might never use, then you could do what I have done....I wrote it all up in a thread here -
http://androidforums.com/x10-all-th...-x10-setting-up-boot-loader-custom-theme.html
I basically upgraded to Android 2.1 and still found the phone sluggish, so I rooted, installed a new bootloader...did a backup of my current image, then installed a custom image which was streamlined and had a nice custom theme....I still messed with it a bit after that, but that was more superficial stuff like changing icons and widgets and stuff after.
I can still go back to normal 2.1 that my cellular provider gave me....but doubt I will...so much quicker now.
