Robot speed varies throughout the operation time

We just installed IRB 4600-40/2.55
3000-310 Controller variant: OmniCore V250XT

3026-2 EtherNet/IP Safety Device: CIP Safety Adapter

While the robot is in operational mode. Cycle time varies between 35 seconds to 39 seconds. There is quick acceleration after robot runs for a while about an hour or two later.

What can I ask my automation team to look into to make it a consistent operational run?

Best regards,

Niraj