The conclusion about the FP read/write blocks in my earlier message was not correct, I misinterpreted the profile data.
Because I could not conclude anything after analyzing the profile data, I took a different approach. I have cut down the program until the CPU time was not increasing anymore.
After having found which while loop caused the CPU load to increase, I cut out only this loop. Then I tested the program in that state and there was no increase of CPU load to be measured for half an hour.
Then I have put in the while loop again. It was a loop in which I determined the state of a few buttons and the fill color of a Thermometer. So I cut out every link (read and write) to attributes in that loop and ran the program again. There was no increasing load!!!
So I concluded that it had something to do with the attribute reading/writing. To see what happened I put the loop as a stand-alone in a new VI and ran it. No increasing load for over an hour!
Now I am puzzled; How can it be that a while loop causes my program to get increasing CPU load, but not as a stand-alone? I have already checked if I was writing to the same attribute in different places, that was not the case.