When IDM finishes downloading it takes a very long time to rebuild temporary files. Why can this happen?
The problem is 100% caused by conflicts between IDM and other applications that try to control all disk activity. It can be some FireWall/Antivirus/InternetSecurity application for example, which controls traffic and all files (file parts) that are written on disk. They always go-between IDM and real file on hard drive two times (for read and write processes) and can slow down the whole process greatly.
Maybe you have some application that has such function and you do not even know about it. Also, please note that some Antiviruses and other security applications keep controlling everything even if you disable them because they still have their drivers in memory which do this job in background. This why, you need to uninstall them TEMPORARY and check if problem becomes any better. If it does not help, look for another application on your computer. It may have something like "Protector", "Defender", "Spyware Remover" or "Cleaner" in its name.
In addition, some FireWalls and Antiviruses may be not ready for Windows Vista, Windows 7, Windows 8/8.1 or Windows 10 and work incorrectly. Or maybe some application is stable with Win 32 bit system but not fully compatible with Win 64 bit system.
Also, it is possible if you have an old IDE drive that is slow. In such case, open "Options→Connection" IDM dialog (arrow 1 on the image) and set 1 as "Default max conn number" parameter (arrow 2 on the image). Then press "OK" button (arrow 3 on the image). In addition, temporary folder and folder where you want to save a file should be on the same drive. Then IDM will not rebuild any temporary files, instead it will simply move a downloaded file into destination folder.