PerfProtector writes "Did you recently install Thunderbird 3 or upgrade from Thunderbird 2 to Thunderbird 3? Did you notice any severe slowdown in your machine or a major decrease in its performance? Well, many people around the world encountered these problems. We wrote a technical analysis about the severe problems that are caused by Mozilla Thunderbird e-mail client. These problems include anomalous usage of CPU, memory, hard disk and Internet bandwidth. You can read the full analysis, including several graphs that show how bad the situation is and what went wrong from Thunderbird 2 to Thunderbird 3. For example, while CPU utilization of Thunderbird 2 is usually between 0% to 10%, with an average of 0.3%, Thunderbird 3 CPU utilization is between 5% to 80%, with an average of 30% — 100 times more than Thunderbird 2. In addition, during long periods of time, Thunderbird 3 used more than 50% of the overall CPU resources.This behavior slows dramatically the whole machine."
It's worth noting that this analysis comes from developers who have developed a (freeware) tool they claim will improve Thunderbird's performance, but they explain also how to do so with manual changes.