Reducing the AP to only using B rates is actually the opposite of what you should do.
Here's the math:
An 802.11g AP has a max PHY rate of 54Mbps, but 802.11a/b/g efficiency for TCP/IP is only about 50%, so at max, each AP can serve up about 27Mbps of bandwidth. And you're splitting it 10 ways, so under perfect conditions and perfect fairness, if all the clients were doing a big download at the same time, each client would get about 2.7Mbps.
When you said you measured 500kB/s, if you mean KibiBytes/sec (1,024's of 8-bit bytes per second, a.k.a. KiB/s), that's about 4.1Mbps (1,000,000's of 1-bit bits per second). So obviously that's better than my "if all clients were downloading at the same time" estimate above, since surely while you were doing this test some clients were mostly idle. So it indicates that your performance is within the ballpark of what can be expected from 10 clients on an 802.11g AP.
An 802.11b AP has a max PHY rate of 11Mbps, so with that same 50% efficiency, that's 5.5Mbps, split 10 ways for 550kbps (1,000's of 1-bit bits per second) each.
Why is your client still messing around with 11-year-old networking gear? It's time to upgrade! Your client obviously likes to keep gear around for a really long time, so it's best to fully modernize them right now so they won't be as obsolete as soon.
1300Mbps-capable 802.11ac USB 3.0 adaptors are about $70 right now.
1300Mbps-capable 802.11ac APs are about $180 right now.
So for ($70 * 30 clients) + ($180 * 3 APs) = $2640, you could fully modernize that network.
Even in realistic conditions, you would have 4/5/600 Mbps of bandwidth per AP to share, so each client would have 40/50/60 Mbps of bandwidth, so your test that used to be 500KiBytes/sec could well jump up to 10MiBytes/sec.
Actually, the picture could be even better than that, because my calculation above was based on only using the 5GHz band. But all 802.11ac APs today are dual-band concurrent, so they're each effectively two APs in one box. So you could leave 1/4 to 1/3 of your clients on the 2.4GHz band for even more aggregate throughput.