Iksar have a 20% xp penalty (like trolls), see
http://www.rightmove.co.uk/property-...-29249950.html
Necro have a 10% xp penalty like every INT caster (can't fins the source).
End result: total xp penalty is 28% (1-0.8*0.9) so yes, an iksar necro will level slower than most other combination (the only worst ones involves hybrid classes (sk, pal and bard).
Now coming to playing one as your first toon, remember one thing: what do you play for? I play for fun and can't give a rat's behind regarding my leveling speed, do you? Once you're at level 60, this won't matter anymore (and xp penalty didn't affect AA xp). Necro is a great class to start with as it's very powerful and will let you enjoy the game solo when you can't find a group but also farm some nice stuff to get some cash. Only you can tell if the class/race combo is for you, I sure enjoy mine
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Please feel free to correct my statement if inaccurate.
Nagash/Petitpas