I'm finding the 640XL a bit slow. My old 625 is a more responsive phone. My guess is it is related the the screen size of the XL. The straight 640 may be faster. But you can't expect everything in a budget phone.
On a more positive note, the Dual SIM capability of the 640XL is awesome. (Mine is not a UK retail single SIM 640XL, it came from Hong Kong via mobicity). Was out of the country for 3 weeks, and had a foreign SIM in there as SIM2, handling all data plus local calls. The UK SIM1 received calls from the UK as normal. Back in the UK, I now have EE as SIM1, which is my contract SIM, and I have giffgaff as SIM2 as pay as you go, solely for their cheap international call rates. Dual SIM is brilliant, and Windows Phone handles it brilliantly.