Startup is the only issue. And it's possible Valve just lets all their computers sleep and doesn't realize how ridiculously slow the cold boot time on Steam is.
Everything else is super zippy. And, hey, it's easy to have responsive clients when there's nothing in them. It's the ultimate optimization.
As someone who architects large scale enterprise server/storage solutions for G500 customers, Valve does not let their "computers" sleep. Valve's back end will be running on a multitude of servers, geo-located and always on, packed to the brim hardware wise. Their issue is software, but most likely the database behind the software and how the Steam back end services communicate to the database. This is probably a combination of actual hardware performance (mainly disk I/O) but most likely network throughput and latency.
Physics basically
It could be a result of a growing and aged architecture, and yes the competition has less data to manage. But data and software doesn't really work like this. Origin for example runs great because it isnt handicapped by being developed in 2004 and have roots that sprawl. As someone who's been a "pc gamer" and building "gaming PC's" since the mid 1980s.. and someone who gamed on PC throughout the 90s and into the aughts.. I was there when Steam launched.
It was hot garbage, nobody in their right mind would argue against that. Steam back then was a store only to sell Half-Life and Half-Life 2 basically. Form, functionality.. pretty much everything about Steam back then was so terrible it actually pissed off the gaming community and Valve was demonized for gating their software behind a shitty DRM store. It took Valve probably 2 years to fix glaring issues and just get basic functionality to work. From there they started to slowly, slowly sell 3rd party software, then more then more. All the while, literally zero competition.
Right place, right time. This is largely why they exploded, not because their client was great software. Imagine Windows XP still running in 2019 and what that would feel like. Thats Steam, lol.
Even today in 2019, Steam doesn't have "real" competition. Origin, GOG, UPlay sure, but even more now with the Epic launcher, Battle.net Launcher and more. But yet, Valve really hasn't been pressured to overhaul much of anything of Steam. They haven't "needed" to yet, which is the only reason. But the net result of that is Steam in 2019 is looks and feels like Steam in 2006, its just dark grey instead of olive drab green. Clearly there have been thousands of updates introducing features and bug fixes but all of this is tossed atop a pile of aging, old technology.
To put it blunt, shopping and using Steam in 2019 is like walking into a Wal-Mart when you can pay a little more to shop at Target across the street and avoid all the mess. Just because Wal-Mart has lower prices and everything in 1 location doesn't make it better.