Could be all manner of things, from the available internet connection capacity, your email service server response time, your own computer/device being busy or needing a cleanup, your anti-virus software interfering with Thunderbird, and more ... In order to make any kind of sensible comment you would need to provide more details, such as what operating system, what version, what anti-virus product, whether you notice a difference when running in safe mode (Thunderbird) and safe mode (Windows), or with anti-virus off or set to exclude Thunderbird/email scanning.