I noticed performance impact from having DLink PCMCIA/CardBus 802.11g devices on my Toshiba Librettos (tiny top-of-the-line original Pentium boxes) as well, I imagine too much processing is happening on the machine-end rather than the card for weaker systems to be comfortable. There's not really any fixing that, though, other than to not use the card when you don't need it for something. Since the machine can't really *use* that much speed at that point it might make more sense to use a 802.11b connection, to be honest, but it's not exactly accessible these days, you kinda have to have that explicitly set up.
The reason I seek an 802.11n card is that that will be supported on networks and network hardware for quite a while longer and it just barely stretches the limits of what 9x supports (it's always fun to push limits even if it doesn't serve a real purpose, at least for me).
There are a number of 802.11g cards that work with 9x, there's a 3com one with a collapsible antenna that sits inside the slot when not in use (and it disables the card when pushed in, which is nice due to the taxing of the hardware to use it from earlier machines in that era) for instance, and lately I've been using D-Link DWL-G650 cards in my Librettos. There are of course PCI cards for desktop users.
If you're cool with 802.11b there's even more options, even for older machines in PCMCIA like the Cisco Aironet 350 as well as ISA<->PCMCIA which I've used with that card on a 486 desktop this way under DOS/WFW - quite a satisfying thing to make work.
One of the issues is even with drivers most of these things don't support modern encryption (still WPA2 somehow even though that's not very secure). Using the Odyssey client solves that if you can find a copy that works and get it configured properly, but the D-Link client works as well. It's all hit-or-miss depending on the software unless you use a third party one like Odyssey.