images still running slow

Hi

I have installed the plug in and I have done the smush of the images but it doesnt seemed to have changed anything?
I have photos on my website already and they are large files so running slow – I read that it should reduce the size of the images already on site is that right?
how can i make sure it is running correctly and that all images are uploading quickly on the website

Thank you

https://www.ads-software.com/plugins/wp-smushit/