Rated 1 out of 5 stars

very slow and opens multiple messages instead of the expected single reply message

This review is for a previous version of the add-on (1.0.4). 

Slow in IMAP, can't do anything for this. If there is multiple messages, it's probably because you have virtual folders or filters that copy your replying message in several places ! Further improvement could be an option to only display once a message with a specified ID, but the add-on can't invent which copy of the message you search !