It sounds like someone has possibly fitted an alien spring, or the spring is in two pieces.
If there is any lubrication in the spring guide, it will be via the spring.
It's always worth a light coating of lubricant on the spring when re-assembling the pistol.
The phosphor bronze piston ring gets it's lubrication from the few drops of SAE 30 sparingly applied through the cocking slot, as marked.
I don't think I've ever chrono'd a standard Webley pistol, as I can normally tell if something isn't right from the feel of it when cocked and fired.
I'm sure you'll get there in the end.