The slow trains blocking fast trains is a waste of money and time, but luckily there are strategies to fix this, this is one way
A train coming from the green line will reserve the path first (using path based signal), so the one on the red will have to wait. However, if the train on the red arrives soon enough and reseves the path first, the one on the green will take the track at the top with the signal, to make sure it does not have to completely stop long before the junction. The length of that piece of track depends on the length and speed of your trains.
Also you may use just simple block signal`s, but as One-way tracks, use Path Signals for Two-ways