We are in the unfortunate situation of finding and fixing a severe performance regression in back-to-back releases of Hail. In 0.2.19, we fixed a problem that only existed in 0.2.18 and led to extremely slow Table/MatrixTable read steps (large hang time before read started) for tables written in 0.2.18.
Please update to 0.2.19, released today, and see the change log for full patch notes.
Edit for clarification: files generated in 0.2.18 are perfectly usable in 0.2.19 or later. The problem was the reader code in 0.2.18, not the files themselves.