Hello Yip041234,

There are a lot of things that are related to any parallel execution. It would help if you considered the current load, the currently active processes and applications, the implementations used in those applications. OS limitations or undergoing recourse needs, the possibility of a specific task to run in parallel, the needed task execution time and CPU processing time, etc. All of those reasons (and many others) might be interfering with your execution. 

Another essential thing to consider is that Fiddler timestamps are using the OS default system clock by default. On Windows, the system clock precision is around 16ms (set to update 60-100 timers per second), and this is made to preserve power. The system timers are made to be really precise (to save power and due to the fact that the common user didn't really need absolute precision), leading to apparent derivations in the range of milliseconds. You could use the Enable high-precision timers in Fiddler (Tools > Options > General > Enable high-precision timers), but again that won't guarantee that your task can be executed at the same time (in fact, they won't due to the many factors listed above).

As a side note: This forum is for Fiddler Everywhere. You could search for topics related to the classic Fiddler here.