Optimal media type for main file system

I’ve recently installed OpenWrt on a Raspberry Pi and would like to optimise it as much as possible.

My Raspberry Pi uses an inexpensive micro SD card. I’d like to get a better card but I’m wondering which qualities to look for. Some SD cards advertise themselves as having high read/write speeds, the marketing around others suggests they are engineered for long lives.

Which kind of primary disk (in this case the SD card) is it more important for a router to have and why?

My understanding is that most of the information is held in RAM anyway, and since there is 8Gb and only 10 devices it seems unlikely that it will every need to use the swap-space. So should I opt for the long-life disks (if that’s not just marketing) or should I go for the faster one? I’m not worried about the cost.

I use a cheap sd that was already lying around since 4 months and it's still going.

By default only the configuration changes will be written on the disk. Everything else is on ramdisk filesystem. Therefore, unless you install some package which writes in the disk quite often, you should not worry much about the lifespan of the disk.
Also the 8GB ram is more than enough for routing purposes. Once the boot process is done, everything will be loaded in the ram and you won't actually witness much better performance.
I don't buy any expensive, long life, or ultra fast cards. I buy some vfm with long warranty and keep a spare in the drawer to fire up and restore the backup, in case of failure of the primary card.