Using the STM32CubeMx v4.20, I did a webserver with a custom page that show two pictures of about 27KB each. I tested the webserver with the STM32F746G-DISCO , NUCLEO-F746ZG and NUCLEO-F767ZI.
What happens is that when I press F5 to reload the page in the web-browser, most times it takes a long time to reload the pictures. See Wireshark captures in anexo.
When I tested the same page with the same LwIP configuration in the FRDM-K64F board (RMII using KSZ8081RNA), it works ok, I can press F5 very fast and the page always reloads very fast. I also teste in a STM32F407-EVAL board (MII mode) and also worked fine.
I put an external SPI ethernet controller in the NUCLEO-F746ZG and I only changed the ethernetif.c to redirect the packets to the external controller. And the webserver also worked fine, always with fast reloads.
So the problem should be the hardware MAC/PHY of the STM32F7 boards or the CubeMx Ethernet driver.
Did someone also experience this problem on some STM32F7 RMII boards?