I read that ROS isn't an SMB sharing platform due to performance. If this is done on a spare router, only used for this, how poor is performance likely to be?

Bad.

Is that sorta bad, real bad or just bad?

It's simple. If you already have spare router, then test if it's good enough for you. If you don't, probably don't buy it only for this. I'd rather look at some RasPi-like SBC, which may be even cheaper and definitely much more configurable. Because unless I missed some major development, SMB in RouterOS is very basic, you can have multiple shares and multiple users, but it looks like it's still all or nothing, no fine-grained access control or anything. If you're sure that you won't ever need that, then fine, but otherwise it's bad.

OK thank you. So it's more about features than performance? I'll set up a little test, I just wanted to see if there was something obvious to not make the test worthwhile. Thank you!

It's about performance as well. ROS devices are notorious for low speed when working with e.g. USB flash disks, SMB server also requires some CPU power (and low-end device can't provide it).

Think of it this way:

How bad is it to clean your dishes/laundry in your toilet?

Can you do it - yes
Should you do it - no

Think of it this way:

How bad is it to clean your dishes/laundry in your toilet?

Can you do it - yes
Should you do it - no

Depends............. if the toilet has just been cleaned with a scrubber and bleach and fresh water is cycled into the toilet bowl, heck I know some cats that would drink the water.

How bad is it to clean your dishes/laundry in your toilet?

Can you do it - yes
Should you do it - no

Depends............. if the toilet has just been cleaned with a scrubber and bleach and fresh water is cycled into the toilet bowl, heck I know some cats that would drink the water.

If you're considering it, then you might as well shampoo your head and stick it in the toilet and flush. Can you do it - yes you can. Should you do it - sure thing. Should I do it - heck no.

Not on a regular basis MKX, but I wont tell your wife of your strange habits.

Not on a regular basis MKX, but I wont tell your wife of your strange habits.

Well, you can mention to her .. usually she's the one doing flushing

Not on a regular basis MKX, but I wont tell your wife of your strange habits.

Well, you can mention to her .. usually she's the one doing flushing

Well clearly she doesnt want to go to bed next to someone with soap in their hair!!.
Out of curiosity, how many flushes does it take to get all the shampoo out??

Well, you can mention to her .. usually she's the one doing flushing

Well clearly she doesnt want to go to bed next to someone with soap in their hair!!.
Out of curiosity, how many flushes does it take to get all the shampoo out??

I used to joke with my kids about using the "toilet twirl" method of cleaning their hair...

To answer your question, I think it depends on how far your stick your head in.

Out of curiosity, how many flushes does it take to get all the shampoo out??

It depends on hair length ... right after hair cut, a single might do ... just before hair cut three are usually needed
The aspect mentioned by @biomesh plays a big role as well.

Depends............. if the toilet has just been cleaned with a scrubber and bleach and fresh water is cycled into the toilet bowl, heck I know some cats that would drink the water.

What Cat?
You write Cat?

That smells real good, like Italian coffee. One cultured feline thats for sure........... Apparently more couth than MKX....