External hard drive very slow to load after dropping it and can't be repaired with Disk Utility
I hate to be the bearer of bad news, but that external hard disk drive is most likely damaged beyond repair.


  I was using my WD HDD, it fell off my corner.


This is physical damage. No software can fix this.  This is like totaling your car and then trying to fix it by  changing your oil or using better gas.


  I fixed the thing, I plugged it into the Mac and it loads the HDD but
  it's very very very very slow.


It sounds like there was actual damage that you had to repair, but aside from that, there are two components to an external USB drive:


USB controller which is the bridge between the USB bus and the SATA disk drive
The SATA disk drive


From what you describe, the Mac is "seeing" the controller, but when it tries to access it, things go "wrong."  This would be consistent with a damaged hard drive.

The only thing you can realistically do now is hope that you have backups/copies of what was on that drive.