Blaming network for slow speeds. How to determine if it's really the issue.

So one of our clients keep complaining about slow speeds. Nothing specific when I ask exactly what. One of the main issues seems to be O365.

The breakout happens via a L2 link to the DC and goes through a Palo and an ASA. I've had a look for policing shaping and dont see anything on those devices.(Bear in mind im not an expert on either device)

Pings and traceroutes are fine and respond perfectly when tracing from HO to DC or to 8.8.8.8.

This all started to happen once they did a domain migration.

How else can I prove that this isn't the network ?