And if I don't synchronized the bookmarks can Opera also get slower?

Yes.

The more bookmarks you have, the slower Opera will get and the more work Sync will have to do.

And if I don't synchronized the bookmarks can Opera also get slower? I mean if you have many bookmarks (in my case they are now almost 500)

What I didn't really understand is if I must delete the Sync Data folder again even if everything looks right when I am logged in the synchronized browser.

Uncertain. If everything seems right, you can continue as-is. If you run into trouble again at some point, you can delete the whole folder.

And one question more: I have almost 500 bookmarks. I wonder if the computer can be significantly slower when you have a lot of bookmarks? or does this factor not influence the speed of the computer?

The more bookmarks you have, the slower Opera will get and the more work Sync will have to do. This of course depends on the performance of your system too.

If Opera ever feels like it's being super slow, you can always try a test standalone installation. If the test standalone installation runs fine, you can also import your bookmarks into it and see if it then slows down too. Then you'll know is you bookmarks. Then, in that test Opera, you can keep deleting bookmarks and emptying the bookmarks trash to Opera gets fast again. Then, you'll know what the limit is for bookmarks on your system.

It's nothing to worry about normally though.

What I didn't really understand is if I must delete the Sync Data folder again even if everything looks right when I am logged in the synchronized browser.

And one question more: I have almost 500 bookmarks. I wonder if the computer can be significantly slower when you have a lot of bookmarks? or does this factor not influence the speed of the computer?

As for Sync though, you can goto https://www.sync.opera.com and see if all your bookmarks are there or not. They probably are not though.

If they're not there, you can click the account icon on the address bar, goto sync setup, turn off Sync, goto https://www.sync.opera.com and choose "reset passphrase" to wipe out your data on the Sync server. Then, in Opera, you can goto the URL opera://about, take note of the "profile" path, close Opera and delete the "Sync data" folder in the profile folder.

If you do all that and manually restore your bookmarks like you did before, when you log into Sync again, it should then sync your bookmarks fine without making them disappear. But, still export them to an HTML file first just in case.

If your bookmarks are there on the Sync server, skip resetting the passphrase and just do the "Sync Data" folder deletion. That should get your bookmarks back.