FF62 seems to have lag navigating, typing, clicking and scrolling. FF62 announces a script is slowing down Firefox and asks if I want to stop it. If I disable true key script and reboot FF, the problem goes away. This issue didn't occur with FF61.
I upgraded my FF version from 61.0 to 62.0 and I have not seen a popup stating that a script is slowing down my browser. I used True Key and tried a few operations within the browser and the app and did not see any lags compared to True Key on FF 61.0.
This seems to be an isolated issue on your client, I will continue to test my environnment to be really sure but as I said I dont see any issues so far. Have you tried using Chrome and see if you see any speed differences with it?