Back in the early days of PC’s, comparing performance was relatively easy: just look at the processor and the clock speed. A 486/33 (with an Intel 80486 CPU running at 33mHz) will perform calculations about twice as fast as a 486/16.

Figuring out performance from mobile device specifications is a little more opaque these days. It can be much quicker and easier to just run a benchmark. 

For web-based applications, one useful benchmark is WebKit.org’s SunSpider JavaScript benchmark

Running this can give you an idea how much of the performance difference between two different mobile devices or browsers is due to raw JavaScript speed.