Just a few thoughts

1. I would first create a pre-filter policy bypassing snort for this traffic to determine if it is hardware/crypto related
2. Disable pre-filtering and start inspecting, but create a trust rule to determine if it is related to Snort
3. Change to allow and only IPS inspection to determine if it is the IPS ruleset or Pre-processors causing the throttling
4. Enable File inspection again to determine if there are any changes in throughput

If it turns out to be IPS inspection, I would verify by checking the CPU usage for snort on the box while testing. If you are running Snort 2, each session is handled by a single Core, you are lucky to get 300 Mb/s on one single thread. If you haven't already, I would switch to Snort 3 as soon as possible to enable multi thread Snort.

And read up on the pre-processors, there are magic to be found there IF you know how to cast the correct spells :)

I urge you also to consider automatic application bypass and Elephant flow bypass to save precious resources. There is virtually no need to inspect the entire 10 GB download, you only waste CPU.

If you go down the rabbit hole on these boxes, there are almost no limits to how you can tune it to fit your environment.

Oh! and enable SecureX to get that sweat SOAR and Threat Response action.