Node theming through Views or tpl.php files - which one is better for performance?
The two are essentially doing the same work and are going to degrade over load very similarly from my experience. If you look at how views works and builds its content, contextual filters is just aiding the discovery by passing in all the data the view needs. This has to be done one way or another. After that, the view builds the same regardless. 

However, tpl files are greatly useful for code management. If you are really worried about performance, then you need to look into memcached and/or Varnish - both are open source and can be implemented easily on most *nix servers.