Same here on both of my WRVS4400Nv2 routers! Already running the latest firmware: V2.0.0.8-ETSI.
I don't need IPS so have disabled it. However, exactly 24 hours after disabling IPS (or rebooting for that matter), the speed always drops to IPS-like values and will keep at that level! How strange is that?
It doesn't seem to affect all types of traffic, for instance HTTP is affected but FTP still keeps working at the maximum speed.
I would very much like to have it fixed!
To Cisco: it must be easy to reproduce for you, could you please try this on a RVS4000 or WRVS4400N?
By the way, no need to toggle the IPS function: you only have to press the Save button once on the IPS page to get the full speed back, but only for the next 24 hours!