hi, i am using openwrt on archer c60. it has only 8 mb flash. so after installing openwrt i installed some other packages and now it has only 5% ( 80 KB) space left.
my question is, will this low space affect performance?
i didn’t notice any performance drop by the way till now 
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The SoC doesn’t work with flash at run time. It does the work with ram so the flash is only read and loaded in to ram at boot.
The problem will come if you make a lot of config changes that writes more info to the config files, then 80kB will be eaten up quickly.
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Typically older flash is written with 64 kB erase blocks, so 80 kB starts to be near the limit where overlayfs throws the towel into ring, and says that "sorry, read-only".
So, although there is no direct performance hit (as flash is normally not written at all during operations), there may soon be practical problems like you can't make any permanenty config changes...
You should cook up a personal firmware with imagebuilder or auc or attendedsysupgrade, so that your additions get included in the main firmware image and are compressed better. That might free some kBs for you.
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or, what if i use openwrt 19.07?
that is lighter than openwrt 22.03.
In a way, it’s like asking if you can use a 40 year old car seat for your newborn baby. Not safe. 19.07 is long since eol and it is unsupported. It has many known security vulnerabilities and shouldn’t be used in the internet anymore.
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Older OpenWrt versions like 19.07 are in end of life status, that is they do not get any more updates, not even security updates. So it is a pretty risky proposition running such old versions on an internet exposed router that you expect to secure/firewall your internal network. If you are just running OpenWrt on an internal device/appliance inside your your home network behind an up-to-date secure router, this is less important, but if that OpenWrt router is your primary router, thinks hard whether you think that risk is worth it.
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Ahh, actually i am a Newbie. so i dont know how to use openwrt custom firmware build. i think in openwrt there are channel analysis option, which i think takes up some space. can i get a minimal version of openwrt 22.03 for my archer c60 v3?
or it would be very nice if minimal version of openwrt 22 is available for 8 mb flash devices 
you could start by removing the ppp* packages, if they're not required by your ISP, using https://firmware-selector.openwrt.org/
8mb device will be EOLed after the upcoming 23.05 release, due to insufficient flash space.
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If you're looking for easy solutions, get modern hardware with comfortable amounts of flash/ RAM.
The other alternatives are:
- adjust your expectations and realize that the device doesn't have the space to install lots of stuff (depending on the OEM partitioning that might already apply to the default package set)
- take the learning curve of stripping down the image (online imagebuilder, imagebuilder, building from source; as time progresses, you will may end up with having to build from source, to disable more basic features that can't be disabled on a binary level).
>>320 KB free after all package installations/ full configuration set is the absolute minimum I'd personally be (semi-)comfortable with (aiming for at least 800 KB to 1 MB).
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"No, it doesn't affect performance since it's a matter of storage space. Even if the space reaches a safety limit, it could enter read-only mode. However, you're correct in stating that the available storage space shouldn't directly affect the device's performance in terms of processing. As you mentioned, if the system detects that the available space is critically low, it may switch to a 'read-only' mode to protect system files."
Looks like you are fairly early on your 'Openwrt journey'? Here's my advice based on long experience.
Stay with the latest STABLE version. Always. When storage is low, you've few options - move to a new device or create your own firmware. Creating a firmware image from scratch is doable but 'faffy' and time consuming - great if u r a bit techie otherwise "aaaargh! ". The advice above re firmware selector is much much easier and will in most cases deliver what you want.
Or find another device. Do your homework - some are much better in terms of stability (e.g. a WRT3200 and wish I hadn't bothered while a C7 works rock solid every time).
Openwrt is not only about a great WIFI AP. If you intend to use more functionality e.g. Adblock, BANIP, DOH, SQM, Statistics etc, or even Wireguard or OpenVPN, and you can place a separate WIFI (e.g. your C60 in AP mode, with or without Openwrt) elsewhere on your network, then consider a x86 based MiniPC or even an ARM based Pi4 (key thing here - great cpu power, but lower energy consumption and can be left switched on).
The install method here, is a little different than usual but not any harder and will give you more than enough storage space and performance - often at less cost. VPN for example will also benefit from the greater processing power too.
I've learned that the slightly longer route to using Openwrt in this manner pays back the benefits many times over!
Good luck!