why not allow block forwarding without capturing: foo(&) foo(1, 2, &)

It's typically taken for granted that better performance must require higher complexity. But I've often had the experience that making some component of a system faster allows the system as a whole to be simpler

The latest SQLite 3.8.7 alpha version is 50% faster than the 3.7.17 release from 16 months ago. [...] This is 50% faster at the low-level grunt work of moving bits on and off disk and search b-trees. We have achieved this by incorporating hundreds of micro-optimizations. Each micro-optimization might improve the performance by as little as 0.05%. If we get one that improves performance by 0.25%, that is considered a huge win. Each of these optimizations is unmeasurable on a real-world system (we have to use cachegrind to get repeatable run-times) but if you do enough of them, they add up.

Optimization in this case is nothing crazy, just something I neglected while designing the framework.

If a UTF8-encoded Ruby string contains unicode characters, then indexing into that string becomes O(N). This can lead to very bad performance in string_end_with_semicolon?, as it would have to scan through the whole buffer for every single file. This commit fixes it to use UTF32 if there are any non-ascii characters in the files.

What is the point of avoiding the semicolon in concat_javascript_sources

   (intermediate value)(...) is not a function
       at something.source.js:1

   ({
     // other.js
   })()
   (function() {
     // something.js
   })();

And no need to walk backwards through all these strings which is surprisingly inefficient in Ruby.

Since the common problem with concatenating JavaScript files is the lack of semicolons, automatically adding one (that, like Sam said, will then be removed by the minifier if it's unnecessary) seems on the surface to be a perfectly fine speed optimization.

reducing it down to one call significantly speeds up the operation.

I feel like the walk in string_end_with_semicolon? is unnecessarily expensive when having an extra semicolon doesn't invalidate any JavaScript syntax.

The template language's restrictions compared to JavaScript/JSX-built views are part of Svelte's performance story. It's able to optimize things ahead of time that are impossible with dynamic code because of the constraints. Here's a couple tweets from the author about that

It's fast. The Dart VM is highly optimized, and getting faster all the time (for the latest performance numbers, see perf.md). It's much faster than Ruby, and close to par with C++.

In the vast majority of cases there’s nothing wrong about wasted renders. They take so little resources that it is simply undetectable for a human eye. In fact, comparing each component’s props to its previous props shallowly (I’m not even talking about deeply) can be more resource extensive then simply re-rendering the entire subtree.

In some frameworks you may see recommendations to avoid inline event handlers for performance reasons, particularly inside loops. That advice doesn't apply to Svelte — the compiler will always do the right thing, whichever form you choose.