The "original photograph" is whatever you import into Lightroom. If you import a jpeg, it's the jpeg. If you import a raw file, it's the raster preview created from the raw file by de-mosaicing, applying a gamma curve, etc. If you import a PSD, it's the preview that Photoshop embeds in the PSD or TIFF.
The original photograph is represented by the state at the the bottom of the history stack.
How do you know that Lightroom does not modify the original photograph? Because you can always go back to it, no matter how many edits you have applied. When you go back to an earlier history state, Lightroom only needs to remove edits from a chain of parametric edit instructions and re-render what you see on the screen. The re-rendering is done by applying the remaining edits to the original photograph.
If your history has 100 edits, why doesn't Lightroom slow down noticeably? Because Lightroom programmers are taking advantage of the fact that most edits can be collapsed into a single curve. Take a look at
David Barranca's scripts for visualizing parametric edits in Photoshop.