However, I've found what appears to be the culprit as regards the overall performance issues. It's not Dell Streak specific, but seems to relate to many devices with Android 2.1 - Bluetooth Share.
I first noticed it was consuming a lot of battery 30 - 40% over the course of a couple of hours, and that was with Bluetooth switched off. I download OS Monitor (a free and very useful tool) and noticed it was also consuming over 50% CPU, so basically it was slogging the guts out of the Streak. A quick kill of the process restored peace, harmony and a previously unseen, marked overall performance boost, which I guess is understandable given the device now has another 50% CPU available!
I don't know how to prevent it starting again after a reboot (without rooting it), but it's simple enough to kill it again. OS Monitor also has a nice little widget that can sit in the notification window, giving you an instant indication of how hard your processor is being hit. I'd thoroughly recommend it.
I've noticed from Googling that a lot of other people are having this issue, but thought I'd share my particular way of dealing with it in case you're still having problems.