Bug#897220: openshot-qt: preview does not run real-time

[Francesco Poli (wintermute)]

Package: openshot-qt
Version: 2.4.1+dfsg1-1
Severity: important

Hello Debian Multimedia Maintainers!

I am experiencing a really troublesome issue with OpenShot.

I am following the
[tutorial](https://www.openshot.org/static/files/user-guide/quick_tutorial.html)
at the beginning of the official documentation.

After adding some pictures and an audio file, I try to use the
preview window, but the preview is dead slow (and even runs at
variable "speed": the first chunk of video that should last 1 s
took approximately 10 s to be previewed, while the second chunk
that should again last 1 s took more than 30 s to be previewed,
and so forth...).
In other words, the preview does not run in real-time and
no audio is played.

If I click on Fast Forward, the preview runs too fast (as expected,
faster than real-time).

This means that the preview window is completely unusable for me.

While running the preview, the CPU usage is below 1 %, which seems
to suggest that this is not an issue due to limited computing
resources.
On the other hand, the CPU is a 4-core Intel i5:

  $ grep 'model name' /proc/cpuinfo 
  model name      : Intel(R) Core(TM) i5-4590S CPU @ 3.00GHz
  model name      : Intel(R) Core(TM) i5-4590S CPU @ 3.00GHz
  model name      : Intel(R) Core(TM) i5-4590S CPU @ 3.00GHz
  model name      : Intel(R) Core(TM) i5-4590S CPU @ 3.00GHz

and the video card used by Xorg is the Intel integrated graphics.
Does this affect the preview performance, as far as you know?

Please note that, once exported, I have no performance issues
in viewing the movie with, e.g., mpv.

Is there anything I am failing to understand?
Could you please investigate this issue and/or forward my bug report
upstream, as appropriate?

Thanks for your time and for any help you may provide.



-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (800, 'testing'), (500, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.15.0-3-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE=en_US:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages openshot-qt depends on:
ii  fonts-cantarell         0.0.25-4
ii  libjs-jquery            3.2.1-1
ii  python3                 3.6.4-1
ii  python3-openshot        0.1.9+dfsg1-3+b2
ii  python3-pyqt5           5.10.1+dfsg-1
ii  python3-pyqt5.qtsvg     5.10.1+dfsg-1
ii  python3-pyqt5.qtwebkit  5.10.1+dfsg-1
ii  python3-zmq             17.0.0-1

Versions of packages openshot-qt recommends:
ii  blender   2.79.b+dfsg0-1
ii  inkscape  0.92.3-1+b1

Versions of packages openshot-qt suggests:
pn  openshot-qt-doc  <none>

-- no debconf information