My test/work with Cinelerra are always with Proxy enabled (generally without Scaler) and mpeg or mov codec for "fast" editing on my very old Notebook. Then I don't know if what I am writing may have sense for you.

OS: UbuntuStudio_16.04
Project: 1080p@29.97 fps.
Source video: mp4, h264@29.97fps
Proxy Settings: 1/3 (640x360), mpeg, 1800000bps.
Settings: Play every frame= checked

Timeline shows: abcd|12345|ABCD
Two transitions 2sec length: Dissolve and Slide.

Cin_Ub16-20181231-static
1) X11-OpenGL:
Framerate achieved MIN= 25.4
2) X11 (use direct x11 render if possible):
Framerate achieved MIN= 29.3

Cin_Ub16-20190206-static-testing
1) X11-OpenGL:
Framerate achieved MIN= 24.7
2) X11 (use direct x11 render if possible):
Framerate achieved MIN= 28.5

I think that mpeg file (by Proxy)) help a lot my poor Notebook because video resolution is reduced by 1/3 but also GOP=11. In the source video GOP=90. Then I guess that the ffmpeg/Cin engine have work less with more i-frames.
What do you think about?
Thanks. IgorBeg

Note:
The Framerate achieved MIN is the value minimum read in the Setting during three plays