I'd argue that the figcaption can't contain an adequate description in pretty much every case. "Figure 3: results of Benchmark Q, showing X to be 40% faster than Y." is appropriate only for the minimalistic bar chart with only two bars, and already there it's pushing the limits - it would often be appropriate to just say "Figure 3: speed of X and Y in benchmark Q" as the interpretation and conclusions from the two bars is a separate, different thing which could be in the text. Perhaps to the reader it's not relevant that X is 40% faster than Y, but rather that X can do the thing in 5.6 seconds - which is clear from the bar chart, but not from that proposed figcaption.

Let's imagine a more realistic bar chart with the results of benchmark Q, showing the speed difference of X and Y in four different [resolutions/tasks/memory limits/processors/whatever], so 4x2 bars. Should the advantage of X over Y and all the other patterns visible in the chart be described in the figtext? IMHO not. In such a case, a reasonable alternative for that content would be not a sentence but a data table, providing all the information but leaving the interpretation up to the reader - just as for the visual chart.