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.
I'd say put in an ODBC driver. It gets you support for a lot of databases right out the gate, especially if you keep things to a relatively simple subset. That way, someone already running a postgresql or mysql database can plug things in and not need to track another place data is stored.
I'm afraid I'm not very familiar with how go-sqlite3 handles that. I just started learning golang a month ago, so it's all quite new to me.
Maybe someone else will have a better answer.
>Are you considering to add support to other databases?
At the moment, no. Comments aren't exactly big data ;)
Just curious - which databases would you like to see and why (what do they offer over sqlite)?