So I disassembled both the power board and the main board. There were substantial stains on the main board, but the power board was clean. Assembled, reset SMC, reset NVRAM and tested, but the behavior remained unchanged. Fueled by slight frustratio…
Oh my, thank you so much John! After your explanation a malfunctioning power board indeed could cause the issues. I wasn't hunting for specific MHz numbers, but as the Air was showing a disturbing lack of performance, I wanted to find out what's cau…