Yes, I did check and it's really Thunar
My guess is this is happening because of thumbnails
But it also happens when I open folders in other partitions
The problem is that I am working with many images also in that same partitions
I have tried (a moment before I write this) to disable some settings in /etc/xdg/tumbler/tumbler.rc
Then I did log out and log in again
And tomorrow I'll see how it goes
However, if someone has some advice, please don't hesitate to reply here