I am selecting a microSD card for NanoPi R4S rk3399 R6S RK3588S 4G is a great new OpenWrt device - #1213 by walmartshopper and I'm wondering the relationship between storage perf and perf for common openwrt tasks. Such as:
There shouldn't be any impact after the initial startup delay.
Just like with a PC with a mechanical hard drive or SSD, the read/write will depend on what is used.
Not sure if you would notice a difference but anything that is not in RAM and has to be read or written will generally be slower with microSD vs something faster like eMMC or SSD.
A slow SD card might take a couple extra seconds to boot, but it shouldn't make a difference for any of the things you listed. A faster SD card is helpful if you run docker on it, but outside of that it shouldn't matter much.
I have put OpenWrt builds on very old, small, and slow cards on similar devices and they work fine... it's actually a great way to put old cards to use that are not useful otherwise.
Initial flashing or upgrading to new builds will take longer on a slow card. I would say if you have an old card laying around just use it, otherwise a cheap 32GB card from a reputable brand should work fine and can be found for under $10.
The Linux kernel caches as much of the disk into RAM as it can. Memory that is "unused" by processes is used for this cache. Any further disk accesses after the first time a file is read are done to this cache (which is in RAM, so it's faster than an SSD) and then "flushed" in the background to the actual storage at leisure (within limits).
It looks like the NanoPi R4S has 1-4GB of RAM. I don't think you have to worry -- your entire OpenWRT install is probably only going to be megabytes. Those particular common tasks you mention should require processes that use far less than even 1GB of memory total. Pretty much everything is going to stay cached in RAM after the first time you access it.