I finally managed to fix my problem after hours of testing and researching.
It wasnt an issue of the app at all. Still i want to give big thanks to markus for trying to help and sorry for wasting your time since it wasnt related to the lg tv app.
Now to my solution:
I noticed that also other streaming like youtube was slow. So i made a speedtest in the lg browser. And it showed only 10 Mbps although i have a 50 Mbps internet connection. On 2,4 ghz the tv and streaming was much faster which was strange, because 5 ghz should be faster.
After a bit research i found out that many LG TVs have a problem with 5ghz wifi with bigger wifi channel than 100. Checked my router and its autochannel setting really changed the wifi channel to 100 since it became empty recently.
I deacrivated auto channel in router settings and manually set it to channel 36.
And voila, 5 ghz speed with the TV is great again and also 4k videos run smooth within plex.
My TV is a LG 65SJ800, but from what i read a lot other LG TVs have these issues with high wifi channels.
In case you havent tried that yet, definatly worth a try.
