Delete a folder with a lot of files

When I try to use "ls" cmd in the terminal, it also takes over 10 mins and does not display anything.

1. This is not normal. Something is amiss. I'd see the most likely cause as either a bad filesystem or a failing drive. While surprising, it could be a bad spot on the drive just where the directory is. In the case of a failing drive, macOS will try multiple reads to see if it can retrieve the data.

   /Applications/Utilities/Disk Utility

goto the First Aid tab. Should you find problems, you will have to boot in the recovery manager. command + r at boot time before the beep. click on the utilities tab and select disk utility.

High Sierra's Disk Utility

Yosemite's Disk Utility

2. Try a safe boot. It's always a good idea to try safe mode when strange things happen.

   Shutdown your machine. Hold down the shift key. Poweron. The boot up will take longer than normal because the filesystem on the startup drive is being checked and repaired as needed. All about safe mode including what features and apps safe boot leaves out. Safe boot uses a software driver instead of using your machines video hardware. http://support.apple.com/kb/ht1455

unix.stackexchange.com once told me that rsync is the fastes way to delete many files:

mkdir empty_dir
rsync -a --delete empty_dir/    yourdirectory/

Maybe this helps here.