Hi @eklundz,
I hope you are well today and thank you for your questions.
1:
I read in the desc that Smush it removes meta data from images in order to reduce the size.
Does this in any way affect SEO-optimization of the images?
I don’t think it any way affect SEO-optimization of the images.
The Smush.it uses optimization techniques specific to image format to remove unnecessary bytes from image files. It is a “lossless” tool, which means it optimizes the images without changing their look or visual quality.
Smush.it currently keeps all JPG metadata(for example: camera information, audio files, thumbnails, and so on and exif (exchangeable image file format) information.
It doesn’t compress the images files but just removes unnecessary bytes from image file. You will find more information about it on the following page.
https://developer.yahoo.com/yslow/smushit/faq.html
2:
I have a site with thousands of images that I’ve (until now) have saved as 100%, which I’ve learned is a big no no.
What would be the most effective way for me to reduce the size of all those maybe 5000 images as painlessly as possible?
In this case you can just bulk smush all these images by clicking on the button “Bulk Smush all my images” displayed on the following path.
Admin Area -> Media -> WP Smush.it
Best Regards,
Vinod Dalvi