Strange bug with SMPlayer

Hamulus

I installed Lubuntu Desktop on my Orange PI Plus 2E. The system is installed on EMMC.
I tried to use SMPlayer to play videos and have the very strange bug: the video lags in fullscreen mode if the original video size is smaller than the screen.
For exmaple, the the screen resolution is 1280*720.
If I play 720p or even 1080p videos they can be played in fullscreen mode without lags.
But when I open a video with less resolution it can be played without lags only if I open it in window mode at it's natural size or smaller, but when I stretch window larger than video's natural size or enter fullscreen mode, the video lags very significant.

I use X11 as renderer in SMPlayer settings. Other renderers give lags at any window size on all videos and causes heavy CPU overload.
The lags are codec independend: both Xvid avi and MKV have the same bug: only large videos can be played without lags in fullscreen mode, but smaller videos cause lags.

How to fix it?