Housekeeping before upgrading to new OSX
It is certainly possible to backup your drive with Time Machine (or SuperDuper or whatever) wipe it and start fresh. This is something that is commonly done with Windows PCs as they continue to be subject to "bit rot" or the inevitable slowdown of a Windows system after a few years of use.

I, personally, have been just upgrading in place since back when the Macintosh Operating System was called "System #" and have rarely found that a clean install did much except take up a bunch of time. However other folks may have different experiences. It all depends on what you use your Mac for and what is installed on it.

If you have a nice fast USB 3 or Thunderbolt drive you could try installing the OS fresh on that and see if you see any real change (not attributable to it being an external device) in performance when booted from that fresh install.

Note that USB 2 drives will be slower and depending on the chipset and drive used in the USB 3 or Thunderbolt enclosure there will already be a performance hit, so take any results with a grain of salt.

Me, if I had to do a clean install, I would image my boot drive to an external drive. Wipe the partition and install the OS and apps fresh. Then use Migration Assistant to bring over your user settings/documents, leaving the apps installed on the old boot drive behind for the fresh installs on the fresh install of the OS. And allowing Migration Assistant to only bring over your settings and files.