Apparently, I’ve been using the MacBook with crippled fan for over a year. I have noticed a kernel_task using up all processing time. I found out that it was the operating system throttling performance to stop overheating. It does this by running a non-processor intensive task. This task has higher priority than user tasks including the processor-intensive tasks that are heating up the processor. The effect is that the processor temperature is lowered. But also poor system responsiveness and overall performance.
When I found out about this, I concluded that the fan was faulty and opening up the Mac confirmed it. I ordered a replacement fan and today it finallly arrived. I immediately installed it and the Mac promptly sped up. It felt like the same huge speed improvement when I upgraded to 8GB RAM and SSD. A fan is officially the third best upgrade for speeding up your Mac.