From what you've written, it sounds like you need to reduce latency, not increase overall speed. For example, increasing MTU to 1500 will reduce overhead and increase transfer speed. It also increases latency and makes stalls more likely on slow connections. Reducing MTU to 296 does the opposite.
If you're doing wild experiments with settings, I suggest you write down what was changed, why, and what the original settings were. I say that because some undesirable side effects may not become apparent for quite some time. Also, if your problem is mainly with the A1200, double check your serial device settings in Genesis. Maybe you're using some buggy serial.device replacement or your baud rate is a little too high for the A1200. Flow control should be RTS/CTS if at all possible.
Some 34k and 56k modems can be flashed with upgraded firmware that improves performance -- you might check with your modem's manufacturer.
I shall have to try a lower MTU then, even though 296 is normally for 19.2k and under modems.
I'm not doing wild experiments. I'm only changing one thing at a time, so I know what is affecting what. And I'm not screwing with advanced settings, just turning various TCP protocols and settings on and off in Miami. So far, I established I didn't need DHCP or Verify DNS turned on, which only slow my internet log on way down. Plus, Verify DNS was preventlng me from being able to use my ISP's DNS, because apparently they no longer respond to verify queries (Genesis wasn't using these). Also, I tried using T/TCP (the one I mentioned that could potentially speed up browsing)... so far I think it only interfered. Probably my ISP's DNS support it, but I don't know about AWeb. I tried to look it up but AWeb's internal search function is broken.
Yes, my problem is only with the A1200. However, another member had said he was having these same problems with AWeb with OS4, WinUAE, and even on a Linux box. I've been using these same settings for a long time, and they had always worked fine. It wasn't until things started changing on the net that I started having problems, though I was not playing with settings at all at that time (if it ain't broke, don't fix it- but now it is broke). My serial settings are 115200 bps, RTS/CTS, 8N1, and using squirrelserial.device, which I've never known or heard about being at all buggy. (Back when I had tried rates higher than 115200, that didn't seem to work well.)
My modem is 33.6k. Interesting idea, I'll have to see if there's a firmware update possibility.