[becks_benny]

Adding 4 digits is not difficult, but doing this fast enough to be accepted by the captcha system is really hard. I was not kidding when I said that if you need more then 5 seconds per round, you failed and must start again.

[jwestbury]

Five seconds is pretty crazy. It's not just an arithmetic problem, it's a visual recognition problem. There are plenty of ways this can go wrong, for plenty of disability reasons, many of which aren't even obvious to the person trying to solve them.

[8organicbits]

Is there a computer-based solver that can solve these, but always takes longer than five seconds?

[viraptor]

Nah, this seems like a really basic CV problem that should be solvable in milliseconds. The hardest part would be rejecting the sides that don't face up, but even that isn't a huge deal.