How to delete Time Machine Backups that were moved into Recycle Bin Trash?
If the drive is getting close to be being full, TM will free the space automatically.

You can empty the Trash (delete the files) much faster with Terminal command

rm -rf ~/.Trash/*


However, you will probably corrupt your entire backup by deleting specific TM snapshots (folders with datetime in the name). Learn how TM is actually storing the data on the drive (hint: symlinks).