Hey, I've got a couple questions for you since I've been trying to replicate this but don't have nearly as much delay:
1. Are you using any sort of frame gen? The built in one, frame gen mod, lossless scaling, etc. These all add input lag and may be part of the problem.
2. Have you tried changing the graphics settings and testing again? Lower frame rates will have more input lag although not as much as you are experiencing.
3. Could you try these tests in some other aircraft? Some other Airbus aircraft and some from other companies.
All of the controls seem to visually move correctly for me although the controls do feel slightly more delayed than other aircraft. Inibuilds may genuinely not be able to replicate these issues because of different hardware or controls, or it may be how the real aircraft acts since it is easy to compensate, just move your stick to stop a roll slightly earlier than expected. The A350 does a lot to try and make the flight feel as smooth as possible for passengers so it may limit the roll acceleration/deceleration making it take a while to stop a high roll rate and feeling like input lag.
Also as a side note, neither the FAA nor EASA have input latency requirements for commercial aircraft. The "Phase Lag" you talk about is a phenomena found in helicopters. Now, if the controls were bad enough to cause pilot induced oscillations (PIO) it would not be certified. Several people have reported experiencing PIO's in this aircraft but for whatever reason I've not experience this. It may just be control settings or overcontrolling of the plane causing this.