The latest Surface Pro 2s are shipping with a new processor -- only two months after the second-generation device was released.
"Microsoft routinely makes small changes to internal components over the lifetime of a product, based on numerous factors including supply chain partnerships, availability, and value for our customers," Microsoft said to CNET in a statement.
For any customers "experiencing...concerns," Microsoft suggests chatting with tech support here.
Microsoft eschewed both Atom processors and power-efficient Y series Core processors in favor of the faster U series.
The Surface Pro 2's performance as tested back in October by CNET got pretty close to the MacBook Air (June 2013) on certain benchmarks.
It should get even closer now with the new 4300U.
Note: I can also confirm that the Surface Pro 2 is shipping with a new processor. The screen shot below is taken from a recently-shipped Surface Pro 2 that I have been using.