Single insert per transaction is slow with sqlite, but slow is sort of relative. It would be probably be 10 to 80 inserts per second or so, a little faster if you turn off synchronous writes in sqlite.

I would guess most use cases for this relatively simple comment software wouldn't be expecting that many comments anyway.

An agnostic data layer should probably go on your list, but I imagine other stuff would be higher on the list...like a captcha. Spam will likely be the top issue long before performance.