AR300M and AR150 should boot a little faster. But all the routers are pre-installed with a lot of software packages which slows down the boot progress. Seems you only need to tethering your data and there is a lot of kernel modules and software package could be turned off.

If you have some openwrt skills you can try to use the clean packages and then only install necessary packages.

What protocol are you using to tether your data?