I keep my iphone plugged in constantly but it does not sync continuously, only on first connect or when requested. It definitely slows the computer down especially with itunes doing the transcoding to 128kbps. iTunes does crash but this is rare and the slowness isn't significant.
How slow exactly is running like "doo doo"? If it's like what I described, I don't think it's anything especially bad. Itunes is just crap.