Solved by sam-writer in post #5 
              
              
                fastT5 now works with transformers>=4.12.5 (this is the latest version I have tested). It also has some tweaks to the quantization settings that should speed it up a bit more. 
I can confirm it works with byt5 now, and it should work with mt5 as well (except for needing to change the ConditionalGene…