Poor answers to performance problems like this is the number one reason why Firefox lost so much market share to Chrome. Until Mozilla and friends pay attention to user pain, no amount of "creating a new profile" will improve the situation.

Hell, you can search the entirety of the public internet in a fraction of a second, but a few hundred thousand emails locks up your machine for the better part of a minute? Maybe the Rust folks working on parallelism can spend a few sprints giving TB some love.