..processes? Obviously they could nearly all experience an increase in efficiency, so I say transport. In principle, shuffling and relocating material requires zero energy. In practice, energy use and associated emissions arising from transport processes are a major global problem.
..specific mechanisms? If I had to pick one, then it would be fuel cells because I think they have far greater thermodynamic potential than the widespread combustion engines. (But, greater magnitude improvements could generally be obtained by changing to different mechanisms, rather than further optimising individual mechanisms. E.g., more rail.)
..basic physics? "What basic physics could experience an increase in efficiency?" Hmm. "What purple could experience an increase in saltiness?" Nope, that makes no sense, what was it supposed to mean?