A 2nd Gen Intel I7 2630m which can be put into the T420 is faster and more powerful (by about 10%) than the process you get in the Lenovo Carbon X1, which uses a ULV processor. The Carbon does however have a better graphics chip and is a bit thinner and of course lighter, but also MUCH more expensive than just beefing up the T420. See the important note at the end regarding high end ultrabooks*.
While most T420 are dog sh*t slow, it's usually a combination of easy to replace parts that cause it.
The stock ram in them was too little and garbage from day one, and the hard drive which was pedestrian at best in it's day is now not only worn and tired, but insanely slow and inefficient compared to todays SSDs.
Viva, what processor is in your 420?
The 420 is one of the last Thinkpads that you can still swap the processor and those processors are some of the last full power processors as well. With an i7 and 16gb ram and an ssd, they are actually just about as fast and powerful as anything made in the last year or so, most of which are using low voltage processors.
I7 dual core $45
16GB ram for about $70
SSD - $60
Basically for $175 you get a kick-@ss system that's cheap to repair if it breaks. A new OEM battery (not Chinese crap, $35) will get you between 5 and 6 hours of browsing when combined with an SSD and mild power tuning.
The high res screen (1600x900) is a drop in replacement ($55), and a mobo swap can get you a cheap Nvidia graphics card if you want but don't have it. I had one built this way, and while I think the screen and vid card were over-rated, the system was VERY fast, had good battery life and was capable of running Linux really well. It also had one of the best laptop keyboards made, and you can install open source bios. You can even install a quad core I7 processor for under $75. Wireless AC requires a bios mod and probably costs $10. It's probably the most adaptable and hackable laptop ever built.
If you do still want a replacement,
The question becomes, budget, screen size and intended purpose, but honestly, there is actually very little out there today that will make a built T420 look slow. While you will get more battery life, a touch screen and thinner system, in terms of power and speed, you won;t accomplish anything you couldn't do by just beefing up your T420. Oh, and don't forget, the T420 is a matte screen, the touch screen will be glossy.
Note:
*I happen to love the Carbon, but be careful who you buy from, and make sure it can be returned as many suffer coil whine. VERY, VERY loud whine, more like a scream! Some vendors, including Lenovo can be problematic in getting them to fix it, trying to claim it's not a problem or an inconvenience. I dealt with one and you could hear the whine throughout an entire 3500sq ft. home. So you want to buy from someone you can return it without hassle. Keep in mind that it's not the only laptop of it's style and generation with the problem, Dell especially also has a few models with coil whine complaints.