I'm using a ZigBee button to call an automation which notifies via Pushover and Alexa. Works fine.
I'd like to reduce latency and use a physical button. I can use a Pi, ESP32 or similar to respond to a closure and tell HA.
What's the lowest latency input to HA? My MQTT server is separate so I'm thinking an HTTP post direct into HA might be best.
Is there anything faster? HTTP is fast enough in my testing, it's an experiment as much as anything else.
OP asks for "lowest latency" but HTTP is fast enough, and later in the thread says "perceptibly instant" is the goal. HTTP is going to be your slowest option, almost certainly, but even the slowest solution is "instant".
What exactly are you trying to do, OP?
(Esphome seems like the answer you're looking for. It's faster than MQTT and you can program parts of the automation and service management in the ESP hardware. RF is probably the only way to go faster.)
I'm interested in what other people have found to be the fastest way to deliver events into HA. It's been a useful thread.