I know it's quite heavy but for performance, consistency, and attractive rendering (surprisingly important) having tried many options, including my personal run

I know it's quite heavy but for performance, consistency, and attractive rendering (surprisingly important) having tried many options, including my personal runner-up Byword which hasn't been mentioned, I settled on the usual suspect of VS Code. Usually with the Github Markdown extension switched on due to liking their theme, but obviously that isn't a requirement. Open a Markdown file and use your platform's Markdown Preview hotkey (eg Cmd+K,V). Simple.