Unfortunately I don't have any data points on how Signal operates 2G speeds

I have tried to use signal in a location that had voice, but no data (a very old, very rural tower in the middle of nowhere) a few years ago, and signal was completely worthless. Even though it can read and receive SMS, it seems to require a data connection to send..

Yes, everything except voice and SMS requires data

Yes, that would be the case for any OTT messenger (so, anything but SMS).

The encryption protocol is fairly unrelated to how an app handles poor connectivity. It’s a bit like comparing different websites handling poor connectivity differently even though they are all served over HTTP.

I believe their communication layers are fairly different – WhatsApp used to use something based on XMPP at least until a while ago; I'm not sure what Signal uses, but I vaguely remember it being based on HTTP?

Can definitely be, I never investigated in detail, although I used alternative open source clients for both (signal-cli and a long time ago yowsup), so the information should be in there...

I hoped that Signal's use of HTTP was as a fallback in case a direct connection could not be established to use a more "compact" protocol, but I possibly don't remember correctly what I've read...