I have an RT-N11P B1 still crashes without reason whenever I measure network speed or when I use wifi network scanning in the vicinity
What version of OpenWrt are you using?
Per the hardware data page, your device only has 32 MB RAM and 8 MB of flash.
Increased minimum hardware requirements: 8 MB flash, 64 MB RAM
Due to new features being introduced and the general size increase of the Linux kernel, devices now need at least 8 MB of flash and 64 MB of RAM to run a default build of OpenWrt. More flash space is recommended for extensibility, see 8/64 warning:
The reason is you don't have enough RAM.