[orangethirty]

As someone who mostly works with user facing web applications:

1. You will always be playing catch up if your growth takes off. Not an excuse to write your system in BASIC, but enough of a reason to forgo efficiency a little bit. Shipping something that is 90% efficient is better than not shipping something that is 100% efficient. I have lost almost one year on the latter.

2. The importance in a web application project (if you are aiming to use it as a business) is your ability to adapt to what the market wants. If you cannot push out features (inefficient ones), then somebody else who can will swoop in.

3. In terms of numbers: I have built systems that handle 10K/requests per second without a sweat. Using standard boxes and Go. Note that as nice a Go is, its still a young language. A typical python/ruby/php system can be made to handle that, but requires a bit more work. Now, what I tell people is build something safe (as in no SQL injections, XSS, etc) first, then focus on growing it. Picking a good stack early on helps, but don't pick a stack you don't know. Someone who is proficient in PHP should not use Rails (example).

4. The key to web services is to find and build things that people want. How do you that? Pay attention to people on social networks.

If you have a background in C, Go and Python feel the most familiar. Some prefer Ruby, which is another fine language. Be aware of the current trend of everything being written in Javascript. Learn to use the language as you move along. You will need it.

tl;dr

Don't think about business-oriented web apps in terms of efficiency from the start. Use what you know to build something secure and stable, and then if it catches on, scale it.

Like PG said, do things that don't scale.