The Fastrack situation at PortAventura is the worst I've seen at any park, except perhaps for Thorpe during its late-noughties nadir. Seeing Shambhala running one train and alternating between a train's worth from Fastrack and a train's worth from main made me want to break down in despair. Which is sad as the ride is effing fantastic As far as I see it, you can divide parks into four broad categories with regards to Fastrack and operations:
Great operations, no paid Fastrack
This is obviously the most desirable situation, but it's very rare. Off the top of my head, I can only think of the Disney parks (that have a Fastrack system but it's open to all guests for free and is a crowd distribution rather than a moneymaking tool), EP and maybe a few of the smaller parks with decent operations as well (Plopsa? Tripsdrill? Niglo?)
Fastrack, but great operations
The most obvious example of this is Merlin at their best, but Alton Towers nearly all the time. The operations at Towers are nearly always pretty on it, and Fastrack is kept reasonably restricted, so it doesn't affect guest experience too much. Universal and Phantasialand are usually also a good example of this.
Poor operations but no Fastrack
Efteling springs to mind. Usually their ops are pretty poor on the coasters (as anyone who has queued for Joris en de Interminable Wait knows, but at least they don't have any sort of Fastrack service to exacerbate the situation. Drayton Manor is another good example.
Poor operations and Fastrack
Chuck in a low number of rides to that and you've got the Holy Trinity of queue-jump misery. PortAventura and Flamingoland spring to mind here, but only the former seem to sell an enormous amount of Fastrack at a bargain-basement price. At least at Mingoland, it's not particularly well-used, and it's of the 'virtual-Q' type. The Pleasure Beach also falls into this category, but at least they have a ton of rides to soak up the guests.
Most parks fall somewhere between those categories. Obviously quality of operations is a scale, and there are better and worse types of Fastrack, and different allocation rates. PA need to seriously address this issue though if they want to be up there with the top-tier European parks, where they really should be in theory.