I have over 20 years of email (7.18 GB) in my (desktop) Thunderbird profile. I recently tried searching for an old email address and it ran out of steam, processing just a handful of emails every minute. It would take many days for TB to find all the emails at that rate. I opened up the Task Manager and found that the memory used by TB topped out at 909 MB. It appears that once it hits that limit, it must do a memory cleanup to add additional search entries, one at a time, so it slows to a crawl. I have 32 GB of RAM, plus a virtual disk, to load up, if TB wanted to.
Please update the Search Message memory handling to use the multiple gigabytes available in today's computers, not just a single gigabyte.