You may have the wipes wired backwards. The first stop on the switch is slow speed and the second should be twice as fast as the first. If they're running slower, then there may be a wiring issue. A good test is to remove the wiper arms and observe the posts as the wiper motor is running. If it slows on the second stop (high speed) then they may be wired incorrectly.
Also, be sure you have the correct transmission parts. The arms under the dash as well as the pivot arms and bases that mount to the cowl differ between vacuum and electric wipers. If a swap was done from vacuum to electric and the motor was the only change, the system will bind and you'll see results like you noted.
The attached photo shows an electric wiper pivot and base. The vacuum version has a pronounced hump behind the knurled pivot.
