Office 365 Problems

I' m trying to assist a friend with a 111C who is having issues with Office 365. He was previously on BPOS with the 111C in transparent mode (he already had an older Cisco ASA and was phasing in the Fortinet) and it appeared to work okay for him. Recently he transitioned the company to Office 365 and his downloads of email attachments are super slow. He has a 100Mb connection to the internet. We have been doing a lot of testing and have updated 111C to 4.3r7, but that doesn' t seem to help. We reset it to factory defaults and setup a very simple NAT rule with no filtering but the issue persists. Has anyone else seen this? Another very strange component is that if we take a single PC and connect it to his upstream routers (HP 7104dl) we get fast download and upload speeds. But if the Fortinet is in place his download speed is decreased by 80%. The connection almost appears asymmetric. I can' t find any QoS or rate limiting settings that would be doing that. It is very strange. He unfortunately has zero monitoring in place so I' m attempting to get some SNMP tools steup to see if we can gain a greater perspective on what is going on.