Windows 8.1 works a bit faster than Windows 7... so, Windows 8.1 would do better unless what causes Windows 8 to slowdown is not the OS itself but some issue related to malware, fragmented disk, startup programs, to many temp files, etc. Try runing CCleaner, defragging the disk, run a diskcheck (with CHKDSK C: /R typed on a command prompt window oppened as Administrator), and if you have the Windows installation disk, run a SFC /SCANNOW with the disk inserted in the optical drive, and also run a startup repair booting on to the installation disk.
You can run a SFC /SCANNOW even without an installation disk and it will ask for it only if needed... By experience I know it can do minor but important corrections to system file settings...