First, thanks for the numbers, it's useful to see real world examples.
Second, and this isn't meant to be a critique (I'm just trying to understand phenomena I see), is there a reason you prefer presenting it as a percentage decrease? Every time I read "X% decrease" I feel obliged to read the source numbers because I'm never sure if the person is using the terminology correctly or not (you are), since so often people mess that up. For myself, I generally use "X ran in Y% of the time Z took." specifically because I don't want people to misinterpret. Is the "X% decrease" presentation preferred/taught, or considered standard? Am I alone in feeling it's more likely to be misinterpreted?
(Sorry your comment is the one I brought this up on, I've just been wondering this for a while.)
In my experience, it is more typical to use percent change or relative change in the physical sciences and this is how I was taught. Just to be clear, if you have values t1 and t2, the relative change is (t1-t2)/t1. There is a 1 to 1 correspondence with what you described which is t2/t1.
I think teej explained it well. If I say "the new value is +20% or -20%", it is immediately obvious those have the same magnitude and opposite direction. But, for some people, when I say "the new value is 120% or 80% of the old value", it is not immediately obvious that they have the same magnitude. It requires a small extra step for the reader to realize that this means the same amount of relative change.
I always state as relative change. I find that people can get really confused if you were to say "X ran in 110% in the time of Y" even though it is stated in a clear way.
My preferred way of communicating this concept is "we observed a +10% change in X compared to Y." I always use a +/- sign and this helps signal that I'm talking about a relative change.
If I am comparing percents, I'll always specify "relative" or "absolute" change though I prefer to use relative change. Occasionally if the change is small, I will use basis points instead of percents to communicate absolute change.
# time pigz -c /tmp/a.txt > /dev/null
real 0m19.352s user 1m16.148s sys 0m0.344s
# time gzip -c /tmp/a.txt > /dev/null
real 0m47.093s user 0m46.940s sys 0m0.104s