How to fix blurry images on my iPhone Photos app?

I wonder if in your transfer process has resulted in picture files that the iPhone finds awkward to load. What is the format of the pictures--are they jpg, heic, or what?

Can you try converting some pictures to another format and reloading them to see how they behave? For instance, jpgs directly from my Nikon also behave this way, but when I do a 100% (Lossless, I think) conversion jpg to jpg, the new ones are fast and behave normally.

You don't say what "cloud" service you were using.

It sounds like you may have turned on iCloud Photos and Optimize Storage. Optimize Storage saves space on your phone by keeping only smaller, lower resolution pictures there, and relying on the full files being kept at iCloud Photos.  This way you can quickly scroll through all your pictures, even though they don’t take up so much space. Then, when you need the full pictures for printing or editing, etc, the phone grabs the full file from iCloud. This is very cool, but it can lead to a slight delay, sometimes. With Optimize Storage, you can save over 80% of the storage that would be used by Photos otherwise.

You can turn Optimize Storage off at Settings>Apps>Photos by choosing Download Originals.

It may take a while for all your originals to download, and they may not download until you use Edit to force the download for individual pictures.

I've done the conversion three ways, but they are unique to Macs. You should be able to open a picture in a picture editing app and then export the picture as a different jpg or heic. Then upload it to iCloud so it syncs to the iPhone and see if it's better. I think.