The speed with which something loads is as you say, dependent on the connection, and the size of the thing being loaded. The speed at which something executes once it is loaded is another issue. Saying that HTML renders "faster" than flash or an animated gif is like saying you can drive a car faster than you can eat a hamburger. It does not make much sense, but in some particular context it may or may not be "true". Generally, js and flash are used to do things that cannot be done with HTML.
Recommending someone "stay away from" javascript for web-dev is like saying they should stay away from water if they want to learn to swim.
VERY VERY VERY WRONG. I am sure the OP is aware of this, so I won't bother to explain this any further. This is about as ignorant as saying, if you want to paint a wall, use blue paint, because blue is the only colour that will stick to a wall. It is simply not true. Any and all programming languages can be used to access databases and produce dynamic content for an HTTP server.
Very silly. No one wants to maintain a site where all the css and js is mashed together in a single file for no good reason. Totally idiotic. A primary goal of an HTTP server is to minimize the expense of a basic request. Of course, you don't want to put every single js function in its own file, but, generally, no one does that programming anyway. Sometimes js/css/html is compacted in a production environment but not in development.2. Keeping the HTTP requests down will help with this. Add all of your Javascript and CSS together. E.g. All the CSS in a style.css file, and all the Javascript in a JS.js file.
WRT python: python is a server side language. Any language can be used server side, but python is one of the more common ones, along with perl, php, ruby, and some MS technologies I don't know much about. All of those will provide a lot of libraries etc. for doing server side web work. These are very high level activities and "performance" is in part determined by the server and the database (not the language used to interface with the server or database), as is security. So while I might say that perl performs better at text processing (which is what most dynamic web content involves), that is not very meaningful. If python seems sexy to you, use python. It doesn't matter. Except PHP is awkward and ugly, lol.
Graphics are generally a client side event and so your choice of server side language is irrelevant to that. If you want to get started doing web-dev, you have to understand the mandatory client side stuff, which means:
1) HTML
2) CSS
3) javascript
Flash and to a much lesser extent java are also used client side, but for specialized purposes.
If you don't understand the difference between "client side" and "server side" start googling, that is very fundamental.