I’ve just had a really stiff Ultegra front STI, changed cables and everything, still stiff. I removed all parts I could, flushed the living hell out of it with degreaser, then flushed that with water, then WD40. I left it to dry overnight on a warm radiator then squirted the internal with a really thin oil. It looked to me like the factory grease had mixed with road slop over the years and clagged up. Shifts like new now.