If your mailbox hosted in Office 365 and experiencing slowness while accessing the mailbox using outlook, you need to check below things before raising a ticket with Microsoft.
Check the Global DNS Resolution
It is always better to check whether your connection goes the closest regional office 365 datacentre. If your Office 365 tenant hosted in US and you are accessing your mailbox in US but the DNS resolution to Singapore ingress point, then you may to need to contact your ISP to identify/fix the issue.
Check the host name resolution for outlook.office365.com. Below connecting to outlook-in.office365.com as I’m in sitting in India.
NSLOOKUP will show the nearby Ingress endpoint name. Below the ingress point available at office 365 and the location details
Check the network latency using ping or psping test
If you are experiencing slowness in accessing your Office 365 Mailbox, you can check the network latency to outlook.office365.com using ping test, average latency should not exceed 300 ms.
Outlook Connection Status
Check the Outlook Connection Status by focussing the Average Request and Average Response. You can calculate the round trip time using RTT = Average Request – Average Response which has to be less than 300 ms
Trace Route to find the number of hops to reach Microsoft network
Use the tracert command to outlook.office365.com to check the number of hops it is taking to reach the Microsoft network (msn.net)
TCP Idle session
TCP time out has to be configured more than 2 hours on the perimeter devices like firewall and other network devices
check the latency when using Proxy and a direct internet connection
Most of the organization use proxy server to allow the clients to connect to internet. Check the latency when using proxy and without proxy and if proxy shows more latency check if you can bypass it
Disable the outlook Add-Ins to see if any Add-Ins is causing a Problem.
Outlook -> File -> Options -> AddIns -> Manage Add-Ins
Also you can use SARA Tool to find any issues in Outlook issues
Above are the basic things that we can check to see if any issue at the end user side. If everything looks normal, then raise a ticket with Microsoft 😉