Drop down lists is issues, such as used in custom fields, versions, assigned_to, among others, are part of the html code Redmine sends to the user. If the number of items is too big (thousands of lines), it generates a very inneficient large select in the middle of the html code that redmine sends the user.

This large html is not cacheable by the browser, and it is sent regardless of it being used. It slows down browsing using next/previous issue significantly, as well as when changing issue type, which reloads all fields.

Depending on the number of items in lists, it could be a big performance boost. Redmine would fetch list items independently of the html, and only when the user clicks or expands a drop down menu. List items could be fetched using a specific route, triggered by javascript, and the select element could then be composed on the fly.