The Preview Frame Rate of a project in the Video Sequence Editor depends on many things:
Speed of your CPU.
Amount/speed of RAM available.
Size of the original image vs that of the project.
Compression scheme (codec) and bitrate of the file.
Speed of the hard drives.
Complexity of the effects applied.
Number of simultaneous streams.
Size of the preview screen.
The first time you play the timeline, blender will decode the original frames form your file, compute whatever transformations you applied to them, and cache the resulting frames into RAM. Some of those operations it can do in real time and most of them it can't, depending on the factors listed above.
Once the timeline has been cached, the timeline should preview in real-time at the project's frame rate (if your machine is indeed capable of that), or at least it should play at a constant speed.
Changes on elements of the timeline strips will invalidate the cache and new frames will need to be recalculated.
Usually you can only cache a few seconds for playback in real-time.
The amount of frames you can cache is limited by the amount of RAM designated in User_Preferences/System/Sequencer-Clip_Editor/Memory_Cache_Limit.
To evaluate complex effects it's recommended to limit the range of frames to preview. Use the P key and drag to create a preview-range. To clear the preview range use AltP or press on the clock icon on the bottom of the VSE or Timeline window.
Playing frames that have not been cached will mean that blender has to calculate things on-the-fly, which might have an impact on performance and result in fluctuating frame rates.
The preview frame rate in the timeline will not affect how the final product will playback. Once you've rendered to a video file you are just playing a single video stream with all of the effects baked into it, meaning that all of the transitions and effects are part of the images, so no more complex computations need to take place, and you should expect a constant playback speed (again, depending on your computer's power and the compression scheme being used).