If Sortd seems to be slowing down here are some things to know and/or to try:
The main Sortd interface runs as a Chrome Extension and in particular as an extension to Gmail and as such is very dependant on the performance of Gmail itself. If you have other Chrome Extensions that extend the Gmail interface its is possible that they will clash and cause performance issues. However as many Chrome Extensions, extend or otherwise enhance the user experience across many websites, it's possible that other Chrome Extensions that are not specific to Gmail may also cause issues, performance or otherwise to Sortd.
If Sortd seems to be slowing down then we recommend disabling your other Chrome Extensions and/or try installing Sortd in a new clean Chrome Profile and verify that Sortd is not slowing down because of a conflict with other Chrome Extensions. Unfortunately due to the huge number of Chrome Extensions available it is not possible for us to test all the possible combinations of Chrome Extensions that may cause problems and generally do not recommend installing or running Chrome Extensions unless you absolutely need and use them. See this link for how to create a clean chrome profile and this link for more detail on Chrome Extensions.
Another thing to be aware of is that the more tasks you create in Sortd the slower Sortd will get. We recommend archiving or deleting tasks that you have completed so that they do not take up space on your boards, as this will mean that Sortd will not try and load the data for those tasks from the server and/or draw the graphical representation of the tasks which will increase the performance of Sortd. Be especially aware of this if you have a 'Done' or 'Completed' list on your board. Rather mark a task as Completed, as it will still be available on your board for a period of 30 days (using the Filters to see completed items) after which it will be automatically archived. This allows you to see and measure progress without keeping clutter on your board for too long.
We are constantly improving the performance of Sortd, so wherever possible we try to minimise the impact of having many tasks on your boards, but if you feel that you are seeing a particular performance issue please reach out to us via support and we will try and investigate the issue.