Slow performance using the original 1TB 7200 RPM hard disk. Installed a 500 GB WD Blue SSD to work alongside the HDD (custom Fusion Drive).
Process was simple. Remove screen, remove superdrive, remove mainboard, install SSD and necessary cables, and reinstall. No issues whatsoever.
Take your time, take photos if necessary for reference, and be gentle on all the small plastic connectors. Get a can of compressed air while everything is apart and give it a complete cleaning before you reassemble.