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In the latest version of SmarterMail 16 the speeds are greatly improved. I want to propose a perceived user experience (UX) improvement which will probably also be a true speed increase for users.
When in the web interface and you drag and drop into a folder or delete a single message or multiple messages the entire list of messages FLASHES due to what appears to be an entire page post back. The same is true when a new folder is created, moved, etc. I proposed that the entire screen refresh / post back be eliminated. Even if the time is the same a user will be happier with that experience.
I just tried this in Yahoo, GMail, Office 365, Hotmail/Outlook.com, and all work this way with no full post back refresh. This tells me it is truly possible to achieve. Personally I know I would find this a truly great experience.