There's been reports of bad performance with Reaktek cards/drivers. Seems Realtek uses the host CPU for more functionality then other network cards/drivers.
There are a number things at play here. One is that there are many 1 Gbs Realtek chips (at least 7, that I know about, but I am sure there are more). Some work better than others (they are used, only because they are cheap). There are also about 4 OS/2 drivers, that I know about. The only one that should be used, is the latest Multimac MMRE driver, from the Arca Noae driver pack. Most of the time, the Multimac R8169 driver will work, but not always. The Multimac R8168 driver can do some nasty things, and the old rtgnda13 driver isn't complete (but it may work, when nothing else does).
Then, there is a problem, in OS/2, where the indicated speed is always 100 Mbs, when it should be 1 Gbs. Some programs seem to follow that, and throttle to 100 Mbs, while others seem to ignore it, and will go as fast as possible (which is always below 1 Gbs). It is a known problem, with no solution, yet (that I know about). This may actually be what causes the test to be slower in OS/2. In real life, you will probably see the same speed on downloads and uploads as windows does, as long as the program ignores the 100 Mbs speed limit.
I am not 100% sure, but it appears that the UDP protocol is very slow in OS/2, while the TCP protocol is actually faster than what windows will do. It is possible that the test is using the UDP protocol. It is also obvious, that windows is far more aggressive at taking it's turn at using the network, and that may have something to do with it too.
Okay, that covers the speed test, but in real life, I have found that most internet sites throttle their speeds to about 300 KB/s (or about 3 Mbs), so most of the time, you will see that, when you are surfing the net.