I would like to create a nice outside light for my interior scene, but without using hdri image (the rendering time increases a lot on my computer and most of all I can't find the way for correctly positioning the sun from hdri). What's the best solution? Should I use a jpg image or sky background and then positioning a sun light source?
The problem with your question is that you are erroneously thinking that there are areas with lights and some without in a JPEG or other display referred image.
In reality, every single pixel is an emitter of light. As you may guess, with all of the crazy math involved in a ray tracing engine, all of that light ends up with a very complex interaction series in any image. This means that it is impossible to simulate real-world lighting without using some form of IBL.
Worse still, a JPEG is what is known as a display referred image. That is, the encoding of the image takes all of the data of a scene and maps it into the display referred range of 0.0 to 1.0. During this transition, you are losing a tremendous amount of the information in a scene.
The most ideal format for IBL is of course EXR. This format permits storage of scene referred values from zero to infinity, which helps to encompass the complexity of most captured scenes.
 For those interested in the nerd stuff behind this statement, there is a great discussion of encoding quantization here.
You can use portals to reduce your render time and get rid of noise in your scene while using HDR images. Here is a tutorial about using portals.