Abysmal Upload speeds to the Cloud and other sources

Okay Networking Reddit Community, I have a dozy for you!

We have a 500Mb symmetrical pipe with Cox Com. and we are having problems with our offsite backups to Google Cloud. The issue started about a week ago when we noticed our backups were failing because they were stumbling over themselves. Keeping it short, 2 weeks ago our backups were running on Sunday and maxed out our 500Mb upload which is what we want. But this past Sunday the backups were capping out at 100Mbps.

We have run speed tests with Ookla and Iperf3 to test the connections from different machines and servers in our network. We get different results all over the place. Which I understand speed tests can be unreliable when wanting to know true Up and Down rates. So we tried pushing some very large ISO's to Google to see what and actual file would do when trying to upload. When doing the actual file upload we can only get 100Mbps upload. It is like there is a brick wall and that is it.

I used the same file and from my house I can upload at 700Mbps, yes I have fiber, to the same Google instance that we store our backups. So I am thinking that it is not Google capping me on the upload speeds.

Does anyone have any Ideas that could be causing the 100Mbps capping for the backups. And before anyone asks:

• We have disabled all traffic shaping policies on our Fortigate and all the interfaces
• We have run and Iperf test from the Fortigate and there is no issues with Up/Down speeds
• We have also tested the same file uploading from different Servers and workstations and get the same result.

Any suggestions or insight would be awesome and I will check and report back. I just love troubleshooting. Thanks ahead of time. If you need any specifics on network design just ask.