It sounds like webOS has built in DLNA Renderer that is active by default.
I did not say to use LG DLNA App. ... you will have to use TV remote to control playback ...
But if you have to use the TV Remote, the TV is running an App of some sort, unless the player is also built into the OS.
Once played ... TV will launch its player and play the file directly, bypassing your PC completely
But it sounds like the Player is an App on webOS.
My Sony works the same way. I appears to have some control layer outside the Video and Music App, which presents itself to the world as a DLNA Renderer. But once playback is started, the TV selects either the Video or Music App to play it, and launches that App. Just like BubbleUPnP which can use an external Player for actual playback. Neither Sony App is particularly good, and neither use the full media playback capabilities of the TV. The DLNA playback capabilities are a subset of the full direct play via HDMI capabilities. That seems to be consistent across all DLNA devices.
So, directing play from MC to the TV via the network and a Zone in MC is using the DLNA App on the LG TV, just like my Sony I believe. Even if it is a layer outside the Player App. That layer restricts payback capabilities, as it presents to the DLNA Server what is it capable of, and that is less that what the TV is capable of using playback via HDMI. The DRMA shows what the DLNA Renderer says it can play, for comparison.
Manfred, Wer's advice is good. Or it may be time to upgrade that video card. More modern cards have better capabilities in addition to more processing power. I am running a GTX 1060 6GB on my HTPC, and I consider that the minimum requirement for MC on a HTPC these days.