The ComboUpdate is a patch for an existing version of macOS 10.11 and not the full macOS 10.11 installer which is needed for reinstalling macOS. This Apple article contains links to various macOS installers as well as optional instructions for creating & using a bootable macOS USB installer:
How to create a bootable installer for macOS - Apple Support
I highly recommend you create the bootable macOS 10.11 USB installer.
Are you planning on reinstalling macOS over top of itself or a clean install by first erasing the drive?
The two most likely reasons for performance issues is from a failing hard drive or some third party software that is installed.
Try running the Apple Diagnostics to see if any hardware issues are detected. Unfortunately the diagnostics don't detect most drive failures. To better check the health of the hard drive run DriveDx and post the complete text report here using the "Additional Text" icon which looks like a piece of paper.
To look for possible software issues run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.
It is best to do these checks before attempting to reinstall macOS.
