It really, really, really can. The example that inspired this for me was the Samarkhand node, where I was losing a substantial amount of my Indian trade, which I was trying to push upwards towards the Baltic to just three traders pushing towards Persia.
The only way round it at the moment is forcing trade power treaties on people - which you don't really want in some circumstances, as it will give you power where you can't use it and take up a relations slot - or by embargoing, which is useless in most cases, as you need to embargo several small traders for a substantial loss of trade efficiency.