On-Demand Loading
From Audacity Manual
Therefore by default Audacity uses a very quick method of importing uncompressed files called "On-Demand Loading". This reads the audio data directly from the source file and allows playing and editing the track almost immediately from any chosen point, while the waveform image is still being drawn.
Features
On-Demand Loading is used for uncompressed files when the Import / Export Preferences are set to "Read uncompressed audio files directly from the original" (in the "When importing audio files" section at the top):
Uncompressed files shorter than 30 seconds long will always import via the normal progress bar, even if Preferences are set to "read directly".
Note: If the above preference is set to "read directly", Audacity will not make a copy of the imported WAV or AIFF. If you intend to save an Audacity Project, do not rename, move or delete any WAV or AIFF files imported when "read directly" was set, otherwise that project will be missing audio.
When you import your uncompressed WAV or AIFF file by any means (drag-and-drop, waveform. The blocks are then progressively replaced from left to right by the waveform image as soon as it has been computed. You can play or edit the track at once from the start of the track, even before the waveform image has been fully drawn.
or ), a sequence of patterned blocks appears almost immediately, representing the uncomputedTo change the point from which the waveform is computed, click at the leftmost point from which you want the waveform drawn. In the image below, we clicked to place the cursor in the waveform at time 2 minutes, 40 seconds. The waveform then started being drawn rightwards from the cursor point:
Once the computation point has reached the end of the track, any uncomputed waveform will be drawn from right to left, unless interrupted by clicking elsewhere at an uncomputed point.
The percentage of the waveform that has been computed is displayed on the Status Bar, as shown at bottom left of the image above. When multiple files are being imported, the Status Bar will display the overall percentage of the multiple tracks that has been computed. To make the Status Bar display the percentage completion of a particular track, move the mouse over that track.
Limitations
On-Demand Loading does not currently support compressed formats such as FLAC and MP3, or formats imported via the optional FFmpeg library.
If the drive the file is being imported from is not able to seek rapidly through the file, the On-Demand imported track may be slow to respond when clicking to move the cursor. This will be especially noticeable before the waveform has been fully computed. This is most likely to occur on Mac computers when importing CD tracks from the Finder. It could also affect older external USB drives.