The number of users split too not just the network, a forked chain with 10% hash power (but with say 10mb blocks) will be used by 0.0001% of bitcoin users, initially? So it wont be congested.
And by the time users catch on, the difficulty readjusts ,and it becomes a standalone chain. It immediately gets added to some lower profile exchanges, then by greed, the big ones add it too, it can become standalone pretty fast,and then we are fucked.
So for the next 20 weeks there have to be brave 10% of miners spending electricity to mine probably worthless coin - please try it yourselves spending electricity for 10% of Bitcoin hashrate (how many MWh ?) and getting basicaly coins worth nothing.
And for the next 20 weeks users going to wait in average almost two hours for each confirmation, sometimes half a day, and if lucky under one hour! There would be no congestion with 10mb blocks, but very long confirmation times for about third of a year.
Its hard to imagine for the first 20 weeks the price of these newly mined coins going to be the same as today Bitcoin price so 10% of miners can continue mining it without burning a lot of their money for the electricity.