They are both based on same sandybridge architecture, i5-2415M has 3MB L2 versus celeron B840's 2MB. i5 also has 4 threads.

So Clock frequency IS the main differentiator here, 1.9Ghz vs 2.3-2.9Ghz (with turbo), which probably corresponds to 30% to 60% difference in general.

Specs of both processors:

Celeron B840: http://ark.intel.com/products/59801/Intel-Celeron-Processor-...

i5-2415M: http://ark.intel.com/products/53449/Intel-Core-i5-2415M-Proc...