> People are suggesting that it's because of the use of cat.

It is:

    > cat /dev/zero | pv >/dev/null
    ^C97GiB 0:00:05 [1.79GiB/s]

    > < /dev/zero pv >/dev/null
    ^C.3GiB 0:00:06 [10.2GiB/s]