To add to what mizuki-chan said it might be a case of low computer specs. Try fiddling with the graphics settings a bit and check to see if you are on the latest GPU drivers.
Another thing you could try is setting a custom framerate.
To do this navigate to C:/Users/(yourusernamehere)/AppData/Local/osu! and edit the file entitled osu!.(computernamehere).cfg
Note that AppData is hidden by default. To unhide it you have to open the windows explorer options, click the view tab and then click "Show hidden files, folders and drives"
To set a custom frame rate scroll through the cfg file until you find the line that says CustomFrameLimit (it should be between the HeightFullscreen and WidthFullscreen lines). This should be set at 240 by default. Change this setting to your liking (I personally have it set on 90 so try that first). Save, start osu and then you should see the option for your custom frame rate in the frame limit select menu