Rewrite photos with better quality

Hello,
1st Issue : We run compression tool in my website and that made the quality of the photos really bad.
Now we stopped it but I need the photos to become the good quality they was before.
I run Sync but nothing happened.
How can we download again the good quality photos and rewrite the old ones? It’s 7000 products.

2nd issue : Plus I have another question. The website is extremely slow. What can you suggest to make it faster?

Many thanks,

Desi