Why do my MBP lag?

Help us help you. The more complete the description of your problem is (what MBA you're using - there are many versions going back to '08, what OS you're using, what Application you're using, a full compete description, the text or screen shot of any error messages) the more assistance we can provide, and, the sooner we can provide it..

One reason could be a full boot volume - you should have 10-20% free space on it for VMEM and swap files, the more free space the better.

Other reasons -corrupt or failing HD, wireless/network signal, over heating, too many open programs, insufficient RAM.

If this answer is acceptable please remember to return and mark it.