The Verizon RAZR is more finicky about chargers and the like than the GSM RAZR (perhaps to reduce aftermarket competition).

A computer USB port is designed to only deliver 2.5 watts (some will deliver more, but this is beyond the specification). Some newer Motorola chargers will deliver more than this, so it may charge faster than via the computer.

If you're frequently topping off the battery (e.g. charging from 2/3 full), this will have less of an impact on charge times because the charging process slows as the battery starts to get full.