A common fix for performance issues with CLASS is to clear your cache. Previously stored data may be interfering with the way CLASS performs as you use it. How to clear your cache:
In the Google Chrome browser, click on the three vertical dots in the top right corner (next to the URL field)
In the drop down box, select More Tools, then Clear Browsing Data
(See below) Select all ticked boxes below to be cleared (selecting, to clear from 'the beginning of time' in the drop down box), then choose CLEAR BROWSING DATA
Once the process is done, exit and reopen Google Chrome and try accessing CLASS.