There is only a exp penalty via race if you do not multiclass or select your race's favored class. Since Half-elfs is any your golden no matter how many multi classes you do. but say you did a elf fighter rogue. Your going to take a little bit longer to level then a Elf wizard.
How to explain it.... On a old server I can see how much exp I got and if the tipical mob gave me 100 exp as a elf wizard but if I went elf fighter instead I think it was between 70-80 exp instead.