files on cloud that takes too long to open

iCloud Drive files live on the Mac, and they are mirrored to iCloud Drive at iCloud.com. If you have Optimize turned on then, to save storage space, you have allowed the Mac to decide to keep some lesser used files off the Mac. Is that you you've done-- chosen to Optimize your iCloud Drive?

As you can see, I don't do that, so I don't have delays. Of course, I do have short delays if I use those files on my iPad, which is optimized. The delays are typically less than a minute-- maybe a couple of minutes for large Keynote files.

Are you needing to keep your files off the Mac? What is your storage on this Mac? At System Settings>General> Storage, my Mac says 745 GB of 1TBB used. What does yours say?

Even if you have your Mac Optimized, you can choose to keep some files local and remove others by right-clicking the file:

In general, to get quick downloads you need good wifi and internet connections. And you may need to leave the device (especially mobile devices) alone so that they don't think that they are slowing down your work by accessing the internet.

See this:

Understanding iCloud Drive - Apple Community

massimo60 wrote: … I have Macbook air with 512 gb and at the moment I'm using almost all of it

Many operations use the iPhone's empty storage space to move files around. If you run out of storage, then those operations may not be able to be completed and, ironically, that can mean that even deleting doesn't work. If you have less than 10% available, then things can get lost forever. This is also true if your iCloud storage space gets low. Until your local storage is reduced below 90%, you'll have slow-downs and other troubles.

On my 1 TB MacAir I found crazy things happening when I had less than 150 GB free, so I try not to use more than 85% of my storage.

So storage is your major problem, and you have to clean that up before you can make any progress. You should use Disk Utility to see if you have Time Machine backups that you'd like to eliminate.

These usually go back only a few days, but they can take up a good bit of space.

View APFS snapshots in Disk Utility on Mac - Apple Support

With only ½TB of storage, you are going to need to rely on external drives to hold much of your data. Optimize Storage can make it cumbersome to make backups, especially for pictures. Here's a way to do that:

Backup iCloud Photos from an Optimized Mac - Apple Community

Thank you very much Richard for taking the time to reply to my post.

I have Macbook air with 512 gb and at the moment I'm using almost all of it as my files are very heavy and I rely on having it on the cloud , but as you heard doesn't matter what internet connection I'm using I always experience very big delay when I try to open a file (even a light pdf ) that is on my cloud.

plus I'm trying now to download a folder from the cloud to an external memory but it doesn't allow me to do so as I can only download a single file and not a folder, if you have and tips for this as well I might be able to solve my problem by having another "back up " on an external drive with the files handy .

thank you so much again for your reply I really appreciate your help

Massimo