Imported PNG image sequences playback with EXTREMELY poor performance when composited (tons of frames drop). Creating a preview render does not fix this issue. Preview renders will still playback poorly as long as the image sequence is on the timeline. This does not occur if the image sequence is encoded in a video file container.
I Can confirm this in version 17.04.3
I am also experiencing this issue. I am using Kdenlive 17.12.0 AppImage on Linux Mint. I wanted to create a timelapse video so I took a whole bunch of JPGs, put them in a Slideshow Clip where each frame plays at 1ms. When I put it on the timeline and play, it is extremely choppy. I tried preview rendering but that didn't help.
Changed "version" to 17.12.0 I can confirm that even on Windwos. Try to rescale the PNG/JPG’s to the video format of the project i.e. 1920x1080. Otherwise Kdenlive has to calculate each PNG/JPG to the project resolution. Change the preview in the timeline (top ant left) from High Quality to Preview or Normal. This change will force Kdenlive to use the MLT previewer instead of the QT previewer.