Just to complete this issue I started, here my final results.

I compiled a clean openwrt version and I used the one provided als “clean” firmware V1.0 form GLI. With both I get a startup time of about 34 seconds on a GL-AR300M. It seems that it would not be easy to reduce it furthermore. So I will stick with “clean” firmware from GLI.

To use tethering over USB I added the packages “kmod-usb-net kmod-usb-net-rndis kmod-usb-net-cdc-ether usbutils udev”. This did not increase the boot time significantly.

So I have a solution for my needs with a boot time of 34 seconds instead of the initial 48 seconds.

So thanks to alzhao for your support.

Regards, Markus