I haven't used Typescript yet, but I'm very keen on it. How fast is the compiler currently for you? (eg. seconds or minutes?, and how big is your codebase? (eg. 10k or 100k?)
I have a Typescript project with over 200k loc and over 200 ts files. The current compiler takes a while to build the whole project, but still under 10 seconds.
However, you can also set it up to compile when you save a single ts file, which is much faster. So in practice its not any slower than what I was doing with Javascript, which was hit ctrl-S to save my changes then quickly hit refresh in Chrome to test and debug.
I haven't tried the new compiler yet, but note that a lot of the compiler's slowdown comes from parsing lib.d.ts each time it's invoked. So the time taken for compiling X lines of code doesn't scale in a linear fashion - a helloworld will probably take a long time for you to compile too, but remove the dependency on lib.d.ts and the same helloworld will compile very much faster.