Not sure what you mean by "lag" when watching TV...you mean if you had another set in the room is teh EyeTV picture "delayed" a little from the cable stream to the other TV? If so, I don't know the answer to that.
As far as watching TV from cable on my iMac through the EyeTV, it looks fine....if you only have 2GB RAM and a slower processor you probably don't want to be doing large DVD rips/conversions in the background at the same time.