Slow Thunderbird, even on my local hard drive

I've just started using Portable Thunderbird and it has really solved a lot of problems for me working between two computers on two sites. However it is slow to open messages and painfully slow to delete them, hanging up the whole program until it's finished the task.
I'm using a Lacie USB2 hard drive and wondered if there was a cacheing delay of some kind (there seems to be some energetic HD or fan action on my office PC every time I do a delete) but I've now tried running PB in a briefcase folder on my laptop's own hard drive and that seems just as slow.

Any ideas?