Mercedes rolled a significant upgrade into Sepang expecting a solid gain. Qualifying delivered a considerably colder shower: Kimi Antonelli fourth, George Russell eighth, both roughly half a second off Verstappen’s pole time.

Team boss Toto Wolff didn’t hide his frustration at the gap between simulation and track: “You’re expecting a 0.3s gain to kick in and it looks like 0.3s or 0.4s backwards.” The trouble started on Friday already — the car was suddenly shedding serious time on the straights, particularly to Verstappen, while the slow-speed Turns 1-2 brought on understeer that cost Antonelli three tenths to the leader in that section alone. “It’s the first time that we’re losing a lot of time on the straights, then through the low speed, Turn 2 we are not able to turn the car,” Wolff explained.

The track itself didn’t do them any favours — Russell likened the Sepang surface to “driving on gravel,” and the aged tarmac combined with extreme heat may well have pushed the new package outside its intended operating window. Antonelli added that the team has always struggled at venues with this kind of open asphalt. Small mercy: he also burned a tyre set in Q2 after a track-limits infringement, costing him roughly one position. The numbers behind the upgrade still point forward — the track just didn’t seem to agree.

Source: The Race (https://www.the-race.com/formula-1/gone-backwards-mercedess-upgrade-letdown-explained/)