Hello! Please, explain, how it happens. I apply “Change speed” with the settinng -50 %, minus. The origiinal track length is for instance3 min, The new length is expected to be 3+1.5=4.5 min. But the real length after applying is …about 6 min, double.
I can guess, the speed it two times less, 50 % of the original. therefore the lenght is two times more.
Right… If you drive at half the original speed it takes twice as long to get there. 
Sometimes the language can be confusing… You could have a change of or by 1% which is a small change to 99% or 101% of the original.
Or you could have a huge change to 1% of the original and the math is different.
The Speed Multiplier is another clue.