In KDENlive configuration, you should see an option to use openGL. Make sure that is on.
Some questions:
What's the resolution of the video you're playing back?
What's the file size of the video (anything in the timeline is in RAM, so if you load a large video into a little bit of RAM...)
What codec? more compression == more decompressing for Kdenlive.
The effects problem is, as far as I can tell, a known issue, and unfortunately I have not heard of a solution yet. The problem is that any effect is being applied in realtime during playback, and that can really slow down playback. My current workaround is to save filters and effects until last, but if that doesn't work for your workflow then the other solution is to do a temporary render of the section of video with the filter on it, and then import that temp render back into your project. Not a very graceful process, but it's what other editors do behind your back, not always with seamless results, so it's not unheard of.
This temp render process is described here:
https://opensource.com/life/15/6/mas...eline-kdenlive (see the "Preview Renders" section)
You might also consider using proxy video for editing ("offline editing"). It's explained in detail here:
https://opensource.com/life/16/1/off...iting-kdenlive