A normal cranking speed is quoted as 150rpm. Determining the speed without instrumentation could be problematical. If speed insufficient to achieve minimum fuel rail pressure, then injection will not be initiated by the ECU.
If cranking speed is considered sluggish, then the fault is basic. The battery may be fully charged, but could have developed a fault. The engine earth strap below LHS headlamp is a weak point on x250 and later models. Replace, unless already done. Check all connections in the starter motor circuit, including the battery earth connection. For safety follow shutdown procedure in handbook, and then remove battery negative while making checks.
If all is good then suspect starter motor.
Thinking outside the box, has there been any loss of coolant? Does engine turn over evenly, or struggle on one cylinder (partially hydrauliced)?