Looking specifically at Portimão, the elevation change has a much bigger impact on the drivers than it does the cars. The undulations create some blind corner apexes, including Turns 8, 11 and 13. This makes it tougher for them to see the entries to the corners and create reference points for braking and turning in.
Because of this, it can make it more difficult to learn the track and build up speed during those initial laps in practice. This is certainly something we found last year, on our first visit to Portimão, where it took a little longer during practice for the drivers to settle in and know where they need to commit to their racing lines and braking points.
Arguably the trickiest complex for the drivers is Turn 10 and 11, because it is a double-right hander with a blind entry - as the track rises by roughly 12 metres - and a steep downhill drop on the exit of 16 metres. The steeper the slope, the more an F1 car wants to become airborne. But it doesn't due to gravity and downforce.
However, the load under the tyres (called the contact patch) reduces as a result and this impacts the grip the driver has on the exit. So, it does make the car a bit more unpredictable at these points on tracks like Portimão, where there is a dramatic drop in gradient.