SlowmoVideo is a free and open source slow motion video making application for Linux. It supported almost all format supported by ffmpeg. You can also load image sequences and it also support the timelapse video with too few frames. It comes with motion blur support and curves that allow arbitrary time accelereation, deceleration and reversal. It is released under GNU General Public License v3.0.
How to Install SlowmoVideo on Ubuntu Linux
SlowmoVideo is available as a flatpak package file from flathub you need to install the latest version of flatpak and flathub on your system and restart it after the installation.
In the terminal app (ctrl+alt+t) run below SlowmoVideo flatpak command. It will install the latest version of SlowmoVideo on your system.
flatpak install flathub net.granjow.slowmovideo
After the installation you can run below command to start SlowmoVideo.
flatpak run net.granjow.slowmovideo
And uninstall it using
sudo flatpak run net.granjow.slowmovideo