I could not find any hardware acceleration option in ChromOS settings. But when i look in chrome flags there are and they are enabled. should they be disabled?
No they should be enabled.
Can you watch 1080p60 youtube videos in fullscreen without problems?
The Asus Chromebook Flip C434 is a pretty new Chromebook model, so I doubt the hardware is insufficient. It is using a fairly modern Intel CPU and graphics solution capable of doing the hardware acceleration required.
On your Windows PC and Chromecast Ultra connected to your TV, are you using a wired Ethernet connection? Or, are you playing those in a location with a better Wi-Fi signal? I know you are almost definitely using Wi-Fi on the Chromebook (unless you are using an adapter for USB), so that is the first thing that comes to mind as a possible difference.
Rule out network issues and then you can focus on any hardware issues, but the raw specs of that Chromebook sound fine to me.