The PLC scan time is usually under 10ms for that model, but the analog input update time might be around 100ms (wag from memory).
EDIT: On the expansion module advanced configuration you can adjust filter settings which affect update times. The fastest for a 1762-IF4 will be 130ms and can be as slow as 530ms. The default is 450ms.
What is the duration of the pulse?
Also, can you share your logic? (zip it and attach to a post)