What is the best KR Server to choose from please ?
No perfect answer for this question because it’s very vary depending on which ISP you’re using or how is the international cable condition good. 
Please note that the credit reduced by the node what you’ve selected so it would calculated whole up and download traffics. That means your credit will double-consuming.
Then please be careful to use Full VPN mode. It would whole of your network traffic through for Mudfish VPN.
Also, Lost Ark has something side-effect to use.
Check this when if you couldn’t connect to Lost Ark