#5 CustomButtons causes delay in Thunderbird 45 startup

[city_zen]

CustomButtons caused the startup time of Thunderbird to go from 2-3 seconds to 15-20 seconds. Moreover, it caused 100% CPU usage during those 15-20 seconds.
I was able to identify that the reason of that behavior is CustomButtons by disabling/reenabling add-ons one by one.
This issue started to appear on Thunderbird 45. On Thunderbird 38 and earlier, CustomButtons worked ok.