What the Speed number is, and is not
Speed is the figure printed at the bottom left of the HUD, beside the Friend Boost line. It is a different attribute from Wing Stamina, which is the bar in the middle of the screen, and from Carry Limit, which decides how many eggs one trip holds. The developer's own description states the progression in one line - Gain +1 Flight Speed as you progress - and the recorded sessions show the number climbing in single steps: 20, 21, 31, 33, 39, 141, 156, 190 and 191 across one session, and 21 to 27 in the earlier one. What Speed is not is a property of your wings: every wing card legible in both recordings stated a Wing Stamina multiplier, and a published wing list for the game states the same, so no source read for this page shows a wing changing Speed.
The first +1 is free at the upgrades stand
The upgrades stand is the stall the game's own banners point at: Get Wing Stamina FREE at the upgrades stand, Now get Speed FREE - outrun the guardians, Claim your Speed FREE. In the later recording the row was read as +1 Velocidad, 20 to 21, with the price cell reading GRATIS, and the claim produced the English toast Increased Speed 20 -> 21! The stand is used with the [E] Mejorar prompt, which is the same prompt the paid rows below are bought through. The recorded session claimed the free step before spending anything on the paid ones, which is the order the banners themselves suggest.
The later +1 steps, and what the rows printed
After the free step the same rows are paid in Cash, with a Robux cell beside them. Four were read in one session: 21 to 22 at $263 with a 3 Robux row, 31 to 32 at $1.06k with a 3 Robux row, 39 to 40 at $3.25k with a 3 Robux row, and 158 to 159 at $54.46B with a 3 Robux row. Each row raises Speed by exactly one point, and the Cash price rises steeply with the level while the Robux row read 3 in all four. These are the rows a single session showed at the moment it showed them; the game prints no per-level price table and no total for the path, so no price ladder is published here. Buy the step you can read in your own session and watch the Speed figure in the HUD move by one.
Speed is counted by the Rebirth panel, and Rebirth takes it back
The Rebirth menu prints a bar whose label is the Speed figure, and in the recorded session it read 21/40 Speed before the Rebirth and 33/40 Speed later in the same menu - a bar that fills as the number rises, not a published rule. That same panel carries a red warning that the reset clears speed, stamina and money, and the session confirmed it: Speed read 33 just before Renacimiento 1 and 20 immediately after, which is the value the game starts a new account with. The menu also offers a second button that skips the Rebirth while keeping the speed. So the honest reading is that Speed is both a requirement you watch fill and a value the reset returns to its base - spend on it knowing which of the two you are doing.
The Speed number did not move after I bought wings
Wings are not where Speed comes from. Every wing card legible in the recordings stated a Wing Stamina multiplier, and a published wing list says the same. If the number you want to move is the one at the bottom left, the row to buy is the +1 Velocidad row at the upgrades stand, which the recorded session reached through the [E] Mejorar prompt.
I bought Speed and still fell out of the sky
A faster flight is still a flight, and Wing Stamina is what ends one. Neither recording shows Speed extending the stamina bar; the bar emptied with the value reading 184/184 while Speed read 190 to 191 around the same time. If the crossing ends rather than merely taking long, the page for that failure is the one to read next, and the fix sits on a different card.
the wing and attribute cards — Check what an upgrade states before you buy it
the Rebirth panel — See the bar the Speed number is counted against
why a crossing ends — Tell a slow flight apart from a flight that fails
the zone labels — Point the faster crossing at an area you have not reached
the payback calculator — Judge an upgrade that buys travel rather than income