How good are they at intercepting another maneuverable aircraft either autonomously or FPV? At what speed is the drone starting to be limited by the human pilot relying on a camera feed?

Unit cost for a lot of these seems to be under ~$5K USD, not counting the value of engineering time.

How do you counter a swarm of these things coming from all directions?

This kind of weapon has interesting consequences for public speaking events by leaders. Or large industrial projects on the coast of Texas that use large tanks of compressed methane and LOX.

This seems like the kind of thing that you can send in the mail to another country in a special box that can open up when it senses that it has arrived at a destination so the drone can fly off to get into position for an attack by hiding itself in some nook on the roof of some nearby industrial building.

Put a small solar panel on it so that it can sit indefinitely, waiting for the signal to strike a target.

Or put a dozen or so of them on an unmanned surface vehicle like the Ukrainians did and send them out to a juicy port target.

The biggest threat that a weapon like this poses isn't just from the initial destructive capacity, it comes from the possible difficulty in attributing the source of the attack.

How do you respond to this kind of weapon you don't know who used it against you?

"Quantum Systems Group reckons it has broken the flight speed record for an electric drone."