I thought that Vlk or Igor said at speeds above 100Mbps.
Since the OP's speeds are way lower than even the 50Mbps if that is the limit at which it kicks in. Then there has to be something else in the mix.
When I installed 6.0.1091 on my other laptop with 512mb, Win XP Pro, Intel Pent 3, 698mhz, My internet speed goes from my 18.0 to 8 to 9.0 When I disabled the 'Web Shield' my speed went up to 18.0
I did a test not just on speedtest.net but using the link in Zyndstoff's signature for AIS ( 77,589KB) for a more real world test and downloaded it using Pale Moons internal download function.
I only timed it using the second hand of my desktop clock:
First, with the web shield enabled and it took 10 seconds, I was surprised.
Second, with the web shield disabled and it took 18 seconds, I was even more surprised to find it almost took twice as long.
All of that without resorting to disabling the Use intelligent stream scanning, weird.