Different response times with different ESP modules

While optimizing the ESP-based temperature and humidity sensors for my home I stumbled upon a weird issue: The different modules show different times to connect to Wifi and send the data. This was a real issue with tha battery driven sensor as there the data was always sent after roundabout 4 seconds. Looking with a 1R and the scope at it, it became clear that the connection/wake time is much longer, even more than 10 seconds.

ESP-12F on an adapter PCB.

A bit could be solved in software. Going to deepsleep after reiception has been confirmed (on:receive-handler) brought the wake-time down close to the self-timed values of the board. Using a static IP borught the self-measured time down to 0.3-0.4s, being really awake for about 1.2s. The timing code used on my other modules revealed something:

ESP-01 old version (512kByte flash, blue PCB): 9-61s
ESP-01 new version (1 MByte flash, black PCB): 3,1s
ESP-12E: 2.95-3.05s
ESP-12F: 4.1s

It doesn't matter at which distance the sensors are to the base station. The connection time is quite stable around those values, only the old ESP-01 boards really vary between those values, most of the times at 15s though. The modules use the same firmware and the same code (one variable differs between the sensors which is their name which gets sent).

It is clear that the first versions may have a different µC stepping and/or worse layout than the newer modules. But I would have expected the ESP-12F to outperform all other boards, which clearly isn't the case. Maybe someone knows the reason for this behaviour. Leave a comment if you do!