If you are experiencing heavy CPU usage with build 3124, try Beta 3126.
Below is a description of how indexing works and reasons you may see Sublime Text using your CPUs.
Sublime Text 3 includes background index workers that scan all source code in folders you have opened, which powers the functionality Goto Definition, Goto Symbol in Project and the Definitions popup. This indexing functionality is not new with build 3124.
When a new version of Sublime Text is installed, it comes with updated versions of syntax definitions for all of the languages supported out-of-the-box. We constantly are working on fixing bugs on these and improving them at sublimehq/Packages. When the syntax definitions change, the indexing functionality must rescan all source files to index all of the functions, classes and other indexed symbols.
By default (prior to build 3125), one low-priority background process is created per CPU core on your machine to index the files. For projects with a few thousand (or less) source code files, this process is usually done in less than a minute. Larger projects that have tens to hundreds of thousands of files may take longer to scan. Some user's may experience the fans on their laptop kick on if all cores are actually doing work.
As of build 3125, we are now defaulting to 1 indexer process for single and dual core machines, 2 for quad core, and n-1 for >4 core machines (desktops). You can also choose the number of background processes you want to run via the
You may also want to check the Console to see if any errors are reported during indexing. It is possible that some source code files may have constructs that the default syntaxes are poor at processing. It may also be that a third-party package has installed a syntax definition that is slow due to extensive uses of regular expressions that cause backtracking.
Please see the directions at http://www.sublimetext.com/docs/3/revert.html if you wish to disable third-party packages for testing.
In the end, it may just be that you need to let the indexer run for a couple of minutes. Once complete, it will only need to scan files that are changed. It will also allow you to take full advantage of the Definitions popup, which I personally find to be a great aid in exploring and navigating code.