Have you tried changing the default Retool outbound region for queries setting?
click your profile avatar in the top right corner, then go to Advanced and look for the Other Options heading:
sorry I can't answer any of the self-hosted questions, I've only used the cloud version, but do your queries use the REST API Resource or maybe workflows? If you're trying to get the best response times before fully committing to the switch I'd suggest seeing if changing one or some to the Run JS Code resource type and handling the fetch call in code manually speeds anything up.... Retool naturally has overhead when using the other resource types, using JS directly can sorta skip over some of this. To go full on efficiency mode you could make a custom component for your API calls and data processing.
you could also try using a service like webhookrelay which lets you make a websocket connection to their service which you can have api calls send their response to. I'm not sure if it'd help or not, but the service is designed for this type of stuff so they probably have a bunch of custom stuff to handle api responses as fast as possible where retool might have overhead for security checks... the response is streamed to you so you wouldn't be adding overhead there. this also requires a custom component tho. I have a post here w the script tag and basic code to setup the websocket