Modern computers are quite fast, and modern cryptographic algorithms are quite efficient.
Most computers benchmark hardware accelerated AES in CTR mode well above 1GB/s, which would be a fraction of a millisecond for a 100KiB file. Since the standard system timer generally runs at 1ms intervals, the entire encryption operation ([file data] XOR [AES] XOR [Pad]) will take less than a single timer 'tick' to complete, and they will report the same completion time.
I would expect files up to 3 MiB will take less than 1ms to complete on computers made in the last 5 years, if the timer is started after the file data is read from disk, just prior to encryption.
Try using a much larger file, at least 1 GiB, in order to gauge the performance of the system. Also, Windows offers a high resolution timer through the kernel, if the hardware supports it, offering sub-microsecond accuracy. If you try to repeatedly measure very small time windows, they will vary greatly if there are other processes using the same CPU core.