Sometimes if there are too many tabs open, or depending on what is open in the tab, it can cause Chromebook performance to slow down. Just by closing out tabs, the performance can increase greatly.
(If there are more than 10 tabs open, you will probably experience some lag regardless of what is open)
Just like with tabs, if there are too many windows open, the performance can slow down. Since windows aren't displayed on the top of the search bar like tabs are, you can click on the chrome icon on the bottom of the screen. Either one window will pop up, or you will see a small box showing the different windows that are open. Clicking on the windows and then closing them out should also help improve the overall speed of your device.