The root of the issue here is when one works with a file or set of files and modifies them regularly, insync will attempt to upload changes every time modification is made. For instance, my several GB worth of temporary files in a video editing session will constantly upload while I’m working on them, and if I’m unlucky, my video editor won’t play nice with other programs uploading the files, (even using shadow copy), and I’m wasting resources starting and restarting uploads.
The OP was offering one solution to this issue, allow a user-defined delay after modification on certain folders. You already allow exclusion of files, which is useful but not a solution to this particular issue. Drop box doesn’t sync temporary files at all, which is not as good a solution as the one offered by the OP.
Another (not necessarily better - it causes other issues) solution is to allow segmented/incremental uploading of files and take care of the sync locally in an rsync-like style, so that only changed portions of the file need to be re-uploaded. You’ve probably thought through these issues, but I figured I’d share the narrative since you’ve been so great at listening to your customers.