With how fast steam deployed this change, it had to have already been in the works for a while (dev time, testing, etc.). The only reason the Steam startup performance improvements are even news is that Epic decided to throw away more of what little credibility they have left to try to make Valve look bad, and failed enormously. Vibe coding isn't competition; it's lazy and a mistake.

So, for steam this was actually available from the 9th of september in a Beta version of the client. And wasnt exactly pushed out as an update to spite epic, it is just part of a much bigger and more interesting update.

Can read the patch notes here - https://steamcommunity.com/games/593110/announcements/detail/710037059469839721

Steam is barely competing, the update come before the Epic boast. Valve is really just doing their own thing.

If they did a day early, it's probably not to beat the competition though

Fuck Epic, let them shoot themselves until they die.