(I apologize for acciddentally sending this e-mail as a reply to you 
personally first, instead to netbsd-users - sorry for that.)

Pavel Cahyna wrote:
>Does lowering vm.bufcache to 5 help?

Maybe.

After setting it to 5, and doing the usual test, twice (unpacking
pkgsrc) I think the jerkiness, while still significant, was not as bad
as before. No long pauses, and the mouse jumps, while still numerous,
seemed to be less frequent.

Then I set vm.bufcache back to 15, and there was the 10 s pause again.

It is really hard to say without using the system for some time, without
doing some longer, more extensive tests, it probably also depends on the
state of the system, applications running... And the mouse pointer is
jerky not only during unpacking the archive, but also when I delete the
pkgsrc directory (which takes a long time)...

And it's not jerky immediately, it takes some time (during unpacking)
before the choppiness begins, probably before the RAM fills up.

But I think it's quite OK now - definitely not good, but acceptable.

Thank you for your time and suggestions.