WP brought speed and lag free operation even when it was a single core experience. From the most expensive to the cheapest handset, you still got a great experience. The same was the case with the dual core bunch and it’s interesting to see that from the main operations, the 925 and even 620 are just as fast as the 1520.
But where the speed and power difference is seen is in third party apps and web browser rendering. Particularly those requiring additional processing power.
620, 1GHz Dual Core, 512MB RAM
925, 1.5GHz Dual Core, 1GB RAM
1520, 2.2GHz Quad Core, 2GB RAM
In reviews of the 1520, one of the main points mentioned was the blazing speed WP seems to be on with this chip. The only “delay” is the transitions (originally used in smartphones to ‘hide’ processing delay, but also useful as ‘signposting’ to tell the user what action will happen next).