The latest version of Shotcut, incorporated a filter that allows for speed changes, slow motion, reverse, fast forward, etc.
There is still not much documentation on some of the peculiarities of this filter.
Here are some discussions on different aspects of the functionality of this Time Remap filter.
Experiments with the Time Remap filter - Made with Shotcut - Shotcut Forum
An example of use.
Note that this filter is a new group and has some limitations in combination with other filters.