At the moment when using the slow motion function the playback is jerky due to the increased time between frames.
I tried a program called SlowMotion from Dynapel that interpolates the required frames to smooth the slow motion out. The results are very impressive when it works but, unfortunately, it is quite old and has not been updated for a long time. The problem is that it now incompatible with most modern systems and programs.
The company was bought up and has disappeared.
The program is still being sold by Goodervideo.com who have some examples on their website of it being used with up to 20 to 1 slow down and still have smooth motion. Attached is a 15 to 1 slowdown taken from their website. I have reduced its length to comply with forum requirements. (Not sure about the copyright so please delete if it is an infringement)
My 'Wish List' request is that this function could be added to Video Editor or become, initially, a separate program, that could be used by extracting the section that is to slowed down which, after processing, could be pasted back into the original film.
This could be incorporated along with michael24's wish for a variable speed addition.