Actually, if you look at the changelogs in the OP, you'll see why the development began to take longer. As I said above, it became more complex.
.2 = 300 renders, 3 animations
.25 = 271 renders, 3 animations
.3 = 232 renders, 4 animations
.35 = 345 renders, 16 animations
.40 = 450 renders, 13 animations
.45 = 713 renders, 16 animations
.50 = 782 renders, 32 animations
.55 = 717 renders, 58 animations
.60 = 703 renders, 47 animations
.70 = 632 renders, 39 animations
.80 = 1194 renders, 51 animations
.90 = 1063 renders, 51 animations
So, that is your explanation. MrJet didn't "slow down" development. He started making more renders, and especially animations. This final update is the same as 2-3 normal updates. Which is why it is taking so long.
See above.