Sound quality will always be an issue with onboard sound. The fact is, sound is affected largely by its parts, and I'm afraid that onboard sound chips aren't made with expensive parts to keep costs down. This causes the Signal To Noise Ratio (SNR) to be lower, which causes a whole range of issues when it comes to sound recording, and sound in general. Examples include with good enough speakers, being able to hear the computer working (i.e picking up hard drive/CPU frequency noises), and hisses.
If the CPU is also working hard in something, and the sound is believed to be a second priority, which it sometimes is when you are listening to audio while doing work for example. You may find the sound will cut out for a second or so, or if the sound is indeed a part of what you are doing, may lag, yes.
Sound cards don't cost much money, and will last a very long time. You don't get any of the problems of onboard sound, since they have their own small CPU's to handle all situations, and tend to be made from better parts.