I see. You may want to read these if you have not already:
Could you share the links to the other projects, please? Also, just like bromagosa said, version hugely matters. There are many other topics in the Forums like this, which seem to be happening due to the version.
This one also got laggy, when I completed the task or mission. But there is a bug(if this project is finished) in the project, which may make it look glitchy.
You could also try Microsoft Edge. Some other coding platforms, like Scratch(not coding language though) have stopped working on Chrome; but still work nicely on Edge. Nevertheless, Firefox should be better.