It's not a 90% speedup, it's ~50% (still quite impressive).
The author seems to be confused, because the original jq is 1.9x slower than the optimized one.
That depends on how you're representing the speedup.
To travel 10 miles, at 60 MPH, takes 10 minutes. Make it 100% faster, at 120 MPH, and that time becomes 5 minutes. Travel just as far in 50% of the time. Or travel just as far 100% faster. The 90% speedup matches the reduction of the time it takes to nearly half (a 90% (projected) speedup, or about a 45% time reduction, as mathed out by kazinator `Projected speedup from both: 4.631/2.431 = 1.905`). Your claim that its closer to 50% is correct from a total time taken perspective, just coming at it from the other direction.
To travel 10 miles, at 60 MPH, takes 10 minutes. Make it 100% faster, at 120 MPH, and that time becomes 5 minutes. Travel just as far in 50% of the time. Or travel just as far 100% faster. The 90% speedup matches the reduction of the time it takes to nearly half (a 90% (projected) speedup, or about a 45% time reduction, as mathed out by kazinator `Projected speedup from both: 4.631/2.431 = 1.905`). Your claim that its closer to 50% is correct from a total time taken perspective, just coming at it from the other direction.