This sounds a bit like:
My car engine does not start. Why??
Please Help!!!!

This won’t work. I can be of no assistance on Windows problems, but other people might be able to, so: Please provide some details:

• Try to describe the kind of lag. When does it lag (starting programs, using a browser, skipping in videos…)?
• What computer?
  • technical specs
  • If you can/want tell: What software is installed?
  • Windows version (press [Windows-key]+[R] and type “winver” in the run window)
  • running on an SSD or HDD?
• Any processes visible with Taskmanager and/or Process Explorer that use a lot of CPU and/or the HDD/SSD?
• What does SMART say about your storage medium if applicable? (CrystalDiskInfo is a quite good choice for Windows if I remember correctly)
• Since when does the problem occur? Do you know what you did right before?
• Did you run a storage benchmark?
  • Do not do this on a HDD which contains data which is not yet backed up! I have seen (failing) HDDs completely die when running heavy load applications like CrystalDiskMark.
• Any indication of malware? Did you run a malware scan from:
  • within the Windows system
  • with another OS (live Linux with anti-malware software)