All sessions captured has been removed.
The highlighted process is the NET Core process of Fiddler (and no the UI). The Windows Garbage collector will clean the memory when needed, but it is hard to say when that would happen as it depends on the available memory (on the PC), the number of the opened sessions from the Saved Session list, etc.
The team is working on improving the overall performance for the next releases. Still, keep in mind that the observed issue should not be a problem as the Windows GC is designed not to clean a memory unless it is explicitly needed.
It also happens on macOS
And it's really slow when I scrolling up/down on the live traffic session list, and the Remove all button is like unresponsive, it take seconds or minutes to take effect.
Learning
After more than 10 hours of use, seems like a memory leak in UI.