Thank you for your post.
First, create a clone of the file (copy with no records). Then, open the clone file. This will let me know if the data is the issue.
When you say "creating a new user account", is this a Windows user account or FileMaker user account. If the latter, create a new Windows user account on the machine, login to the new user account, launch FileMaker Pro, and see if the file opens in a timely manner. If so, then there is something in the original user account causing the issue. If the file still takes a long time to open, then I would uninstall FileMaker Pro 15, remove the FileMaker Pro 15 folder, and then reinstall FileMaker Pro 15, as this would write all new files.
Keep me updated with any progress.