Die Nepomuk ! die !
I dont know and I really don't care if it's disk access, ram or cpu, but experience on a huge range of desktop/laptop shows that
nepomuk on -> computer slow as hell, 'unusable' on any non-high-end computer
nepomuk off -> computer ok
(and of course, not using kde -> computer flies, but this is another topic)
And as said by others, KDE is so well engineered that half of it wont run/compile if you disable any of the fat guys (nepomuk, plasma, ..)
You can usually compile without nepomuk, I don't now if this is supported but it certainly more or less work. But it's moot as its no point. Nepomuk work.
Neposuks and Akoncrappi. Get rid of them both and KDE will be about perfect.
So far I've just been disabling Nepomuk and Akonadi completely. Krunner still launches my programs, shows recent documents and does all the other nifty little things like acting as a quick calculator or launching a web search so I don't miss them too much.
That said, any improvements to them would be good.
Nepomuk has more bugs than kde itself.
It feels nice that a thing that costed 17 million €, you have to disable it forever with each new kde installation.
money well spent.
You know, I must say that I don't remember any real issues I've had with semantic desktop stuff personally since either 4.6 or 4.7 and I use Kontact and Bangarang and such.