Need a lot more information here:
1) Is Office 365 access constantly slow or only during certain periods?
2) Is HTTPS Inspection of Office 365 traffic enabled?
3) How much Internet bandwidth do you have and what is the utilization level?
4) Are other sites slow or just Office 365?
5) When Office 365 is slow, launch a continuous 1300 byte ping to the Internet from the same workstation accessing Office 365, is there any high or wildly varying latency (jitter) or flat-out packet loss?
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