- Is it the cars or the track layout that make it so difficult to overtake?
Both. In general, Formula One cars are hard to follow as a consequence of the vast amount of downforce they're producing.
The wake the cars create increases with the amount of drag they generate. This leads to a proportional loss of dynamic pressure (the air speed onto the car behind) generating a loss of downforce for the following car.
The amount of downforce the following car loses is proportional to both the loss of dynamic pressure and the total amount of downforce of the car. Or, to put it differently: the more downforce the cars have, the more they lose.
On a straight, that creates a bigger tow effect, which makes it easier for the car behind to get alongside the car in front of it. In a medium or high-speed corner, however, this means that the following car loses downforce and therefore grip.
On a track like Baku, with a very long straight and a relatively slow first corner, overtaking is relatively straightforward. But the specific track characteristics in Barcelona make overtaking much more difficult.