I would highly recommend changing your UI resolution to 1080p
Click for more info on Display settings
We recommend people with 4K TV’s set their user interface (UI) to 1080p. Kodi’s UI is not optimized for 4K yet and this can put unnecessary demands on your device and can lead to a suboptimal picture quality, as well as potentially cause other issues.
The settings we recommend are as follows…
Settings>System>Display>Whitelist> (empty) *
Settings>Player>Videos>Adjust display refresh rate> On start/stop
Some televisions may also need, or benefit from, the following being set…
System>Display>Force 4:2:2 colour subsampling> (enable)
With the above settings your UI will be output in Full HD and your 4K content will be output in 4K. *Information regarding the whitelist can be found here. If you have any doubt, feel free to upload some logs so we can verify that your settings are indeed correct.
and to disable the “sync playback to display” player setting.
As for the slowdown it looks like there is an issue accessing your music share. I’m not sure exactly why. I would probably start by double checking your share and file permissions.