There have been many posts describing poor aircraft performance when flying at higher altitudes when the temperatures are wrong, unrealistically high since SU5. There is nothing wrong with the aircraft.
The reason why aircraft are performing poorly is due to Density Altitude which is why this serious problem needs to be fixed.
Density Altitude is the “performance” altitude used by the aircraft and engine. Density Altitude is different than actual altitude because it measures the density of the air particles.
For example, I was flying in a twin engine with a service ceiling, maximum altitude, of about 20,000. I was trying to climb to 18,000 ft but the aircraft was struggling to get over 17,000 ft. The outside air temperature was 31° C. Barometric pressure was 29.85. The density altitude was a little over 24,000 ft at 18,000 ft. There was no way the aircraft with a 20,000 maximum altitude could reach 24,000 ft which meant that 18,000 ft actual altitude was not possible.
Density altitude applies to all aircraft at any altitude all the time. Pilots that fly from high mountain airports during hot summer afternoons should be very familiar with density altitude.