All of my old photos and videos are suddenly blurred

Adonaia wrote: …My old pictures have become blurry after it is backed up on iCloud. I have changed my settings now to Download and Keep Original. How long would it take to restore the original resolution of the photos/videos on my phone? I'm planning to move them all to a hard drive instead to free up some space.

First, something is off-- that's not how Optimize works. All the pictures should look fine. But you didn't tell us what OS you're using, or even if you're talking about a Mac or an iPhone or an iPad, so that makes it hard to know what to do.

When you edit or print a picture with Photos, that forces a download. On a Mac, you can force a download by printing the entire Library as a contact sheet-- cleverly printing to a pdf so you don't use any paper. In fact, it will download the all the pictures before it starts the "printing," so you can stop it before it prints anything. However, this will load up the storage on the device. On whatever device you're using, you need to be sure not to use more than 90% of the storage-- you need at least 10% free to manipulate files.

On a Mac, you can save save the Library to an external drive, even while using Optimize:

Backup iCloud Photos from an Optimized Mac - Apple Community

On a Mac, you can send copies of the pictures to an external drive, even while using Optimize:

Saving Mac Photos to an External Drive fo… - Apple Community

Adonaia wrote:

My old pictures have become blurry after it is backed up on iCloud. I have changed my settings now to Download and Keep Original. How long would it take to restore the original resolution of the photos/videos on my phone? I'm planning to move them all to a hard drive instead to free up some space.

Sorry you didn't do this How to back up your iPhone, iPad, and iPod touch - Apple Support

You enabled this Set up and use iCloud Photos - Apple Support