disktool.dll is a library used by Disktool. This is a system administration software for monitoring the disk and avoid data-file corruption.
Non-system processes like disktool.dll originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that your registry has suffered fragmentation and accumulated invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues.