Is the lag the same with table (if you have one) and mouse?
You should expect, say, a 200 pixel fully hard round brush with 5% spacing to have zero lag. A 1000 pixel brush with 1% spacing is going to test most systems and have some lag. Soft brush presets will lag more than fully hard presets.
If you think you do have an issue, tell us as much as you can about it including your Photoshop version and operating system as well as your system spec.