mp3 thumbnails

Hi, I've read all the posts about thumbnail issues, but didn't find an answer.
So far I only have music files (about 150), the mp3s have a picture in their metatags, each folder has 15 tracks at most. Thumbnails were very slow to be created, sometimes were not. I've enabled ImageMagick.

In each corresponding folder a .filerun.thumbnails was created with a 400_400_cover[1].png and a 400_400_contain[1].png inside, yet some were not showing.

I read what's below and thought that might be the issue, lowered the number a lot, not to have the browser create the thumbnails

After a while all mp3 thumbnails were showing fast when opening a folder.

But when I browse on another computer (or empty browser cache) they're very slow to appear if they appear at all, yet you said on one of the posts that once the thumbnails are created the browser reads them and they're quick to appear.
To see if it improved I've set all thumbnails to go into one folder rather than in each related folder, with the line in the php.ini
$config['path']['thumbnail_cache'] = '/path/with/write/permission/';
Some thumbnails were re-created in that new folder but half of them were not and had that red striked out picture instead.
So I reverted to the "normal" storage place, removing the line.

To sum up, if my browser has already cached the thumbnails it shows them reasonably fast (but still first showing the tracks without any), if the browser is cleared, it is very slow like it's re-scanning and sometimes skips some.
Is that normal behaviour ?