Mr V - the reasons for reduced roll effectiveness at high IAS/Mach numbers are many.
Two primaries being wing torsion as explained and loss of aileron effectiveness due to trans/supersonic airflow effects towards the tips - in some cases outboard ailerons can be almost ineffective. Plenty of reading around if you Google.
You can get deep into theory if you wish and look at modified roll damping in high subsonic flow as well but I suggest you don't
