Is your computer really up to running Windows 10? (Don't think that just because Microsoft said that your PC could handle it that it really can!)
Check your basic system information. (The video below explains how to do it).
The processor speed needs to be 1GHz or higher to run Windows 10 but I certainly wouldn't to try running it with a processor speed below 2GHz.
If the information panel shows that you're running a 32-bit system, the RAM needs to be at least 1GB (but 2GB is probably the minimum for W10 to run well). If you've got a 64-bit system you need at least 2GB of RAM (but 4GB would be better).
Then check how much spare disk space you've got. (Computers will always run slowly when there's only limited space. Windows 10 is a massive download and leaves loads of temporary files on your computer so that you can, if you so choose, revert to W7 or W8, as appropriate). To do so, press the Windows key together with E, then click on 'This PC'.
If you've got very little disk space left you can delete the temporary Windows files to free up some space (but that will mean that you won't be able to go back to W7 or W8).
Many computers that now have W10 on them aren't really well-enough specified for the task and their owners would be well-advised to revert to their older version of Windows. Perhaps yours is one of them?