We are experiencing the same issue, except ours seems to have started with the upgrade to 16.15 (currently on 16.16.5 with no resolution)
When we disable Intrusion detection and prevention, speed tests show 1 - 1.2 Gbps. When it is enabled with any combination of settings, speed tests go down to around 350 x 350 Mbps. Previously IDP would have some minor effect, but we'd get in the 800 - 900 Mbps range.