If your Chromebook has lots of tabs and or windows open, this can cause the device to become slow.
Often the simplest way to resolve performance issue is a simple restart.
Since Google Chrome is always updating and changing, there often will be times that Chrome OS (the operating system for Chromebooks) will need to be updated.
Sometimes data on the clipboard or in the cache just gets to be to much and the Chromebook or device would benefit from clearing the cache.