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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My book "Max Power: Check Point Firewall Performance Optimization"
now available via http://maxpowerfirewalls.com.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones