How fast can a cellist expect to be able to do pizzicato lines, compared to bowing the same line (at maximum speed)? The same speed? Half speed? Slower?

The better you are at playing the cello or whatever at plucking the faster you will be, but you will always be faster at bowings. If you had to pluck at a very fast speed, you would probably have to use more than one finger to do so, but that is very uncommon.