Transfer time machine backups from unmountable HDD
If disk drill won’t show you the directory structure, you’ll need to try a tool like Data Rescue it Disk Warrior so you can find the folder with one specific date to navigate and then restore those specific files. That would copy off at full speed. 


use the folder named Latest  unless you’re sure you want to go back to a specific point in time 


However, if the drive is failing hardware wise, no tool might be able to speed it up, so consider data rescue if you’re not sure it’s software corruption.