What happens when I deactivate

Originals are not preserved during optimization, that is why the plugin defaults to lossless optimization so there is no data loss (unless the moon explodes or something equally unforseen). That said, it’s always good to have backups all the time anyway.

If you use any of the conversion options, originals ARE preserved by default and you can restore them at any time.

Thanks,
I am also using amazon cloudfront. Would there be any issues with a cdn like that.

I don’t know exactly how that integration works, but presumably, cloudfront will re-download any changed images so that it is serving the optimized ones in the future.

So once i install and activate, it will perform the optimization automatically or do I flick the switch? Can I just try it on a few images at first and see how it affects them?

It will do all future images automatically.
Previous images can be optimized all at once via the Bulk Optimize tool in the Media menu. You can also do batches, or one image at a time for testing in the Media Library.

Thanks I’ll give it a try!