I want to use mysterium as proxy (windows)
Could you elaborate on that? You don’t want to run mysterium VPN client but configure your windows or browser to somehow use Mysterium nodes as your proxy? Why using Mysterium VPN client would not work for you?
Some Mysterium servers are slow so I want to proxify only a part of my traffic. I don’t have a problem with running a client app.
This is not possible right now (at least not possible out of the box) but I do believe that if there will be more requests of such kind, there may be created some solution for that.