everytime that happens the file downloaded is empty
That't the reason. {a,e}Mule uses compression whenever appropriate for data transfer. Say, a 5MB empty file is well compressable, it can be compressed to less than 1kB. Now this 1kB is transferred very fast (say, in 1sec), but expands to 5MB - that's why the download speed for the file says a very high speed: you got 5MB in 1sec -> you downloaded with 5MB/sec.
the overall speed in the status string can be two times smaller than my phys-link at the same moment. whom i've to believe to?
You can safely believe the value in the status-line.