Although we can assume since you posted in the Windows 10 forum that you're using Windows 10, but you didn't state anything else about your computer.. Such as: What do you mean by "suddenly it works too slow"? Do you mean it doesn't boot up as quick? Or are you unable to play games as quickly? Or is it simply a problem where internet pages load slowly? Describe the specific problem you're seeing.
How much RAM is installed and how old is the computer? Make and model of computer? The more information you give us, the better informed our answer will be.
Next, because it seems as though the Windows 10 "upgrade" process may not install all the files correctly, or maybe they're damaged, or maybe something else, but running Chkdsk has fixed quite a few of these minor glitches. Try running Chkdsk in "repair" mode to find any damaged files and correct them. Start by opening a command prompt as an admin. RIGHT click on the Start button, choose "Command Prompt (Admin). Once there, type chkdsk /f /r /x, leaving the appropriate spaces between the letters as shown, then press the Enter key. You should see a note that it can't be run till a restart is done, type "Y" for yes, then press the Enter key again. Close the command prompt and restart the computer. Chkdsk will run upon restart and will take quite a while to finish.. Don't panic when the progress bar stays at 10% for a long time. It's fairly normal and the Chkdsk process may take a couple of hours.
When it's finished, it will boot to your normal startup/lock screen and you can login normally and then see if things are working correctly again.
Also, have you tried scanning the computer with antimalware programs such as Malwarebytes, ADWCleaner, etc.? What did you find?
Hope this helps and let us know more.
today i opens my computer but suddenly it works too slow ..what is the reason behind this?