  Cyclic classical and quantum thermal machines show higher efficiency when the
strokes are carried out quasi-statically. Recent theoretical and experimental
work on figures of merit for thermal machines show that they have an advantage
when operating in environments with negative temperatures. In an experimental
proof of concept [Phys. Rev. Lett. 122, 240602 (2019)], it was shown that
quantum Otto engines operating at negative temperatures can exhibit a behavior
in which the faster the cycle is carried out, the higher the efficiency. In
this work, we make use of the concept of entropy production and friction work
to explain this counterintuitive behavior, and we show that it only occurs when
reservoirs have negative temperatures.
