[SOLVED] A friend suggested me the solution. The problem was the FILE CODEC. Smartphones often use H264 codec that are awful to decode on the fly, so preview will hang even with a decent hw. Ironically, a 100 Mb video file from a smartphone (LG G3 in my case) can be heavier to decode on the fly than a 1 Gb file encoded with Xvid (wich probably isn't the best option for a video edit, but at least prevents sluggishness).
I'm stuck in this problem and I don't know if it's a software problem, hardware problem, an OS problem, a codec problem or what.
I'm managing multiple 4k files made with my LG G3.
When I start playing preview, after about ten seconds, FPS value falls down and become sluggish.
Same problem with other movie editors: tested it with OpenShot, KDEnlive et al.
Av-Sync enabled. Frame rate specified. Tried also to re-encode with Handbrake (thought it was a codec problem) but with no luck.
Lubuntu 15.04 64 bit
Installed and running all media codec of this and even the parallel universe
Xeon E3-1241v3 quad core 3.5
16 Gb RAM 1600
VGA MSI GTX 970 oc
... so I cannot believe it's an hw problem.
RAM, GPU and CPU sensors show me that PC is almost bored (near 3% usage, temperatures around 30°C).
Maybe the same old problem with pulseaudio?
So... what can I do?