Thanks again Erel, but let me explain a little bit more the situation I have to live with. I'm working on a board that has already been designed (and produced) so unless there is no other solution, I cannot change the hardware. The Wifi Module is connected to an external MCU via it's UART port. The original goal of the design was to use AT-Commands. The flash memory is on the other MCU (a small STM32) on the SPI bus.
Now I'm thinking of reprogramming the ESP8266 with B4R since I don't think AT-Commands can do the job. Like I said the goal is to download larges images from an HTTP server, transfer them via UART to the other MCU and write them to the external flash. But since the write time to the flash is very slow, I cannot do that in real time.