From various users I've seen discussing the issues, there's been more than half a dozen separate specific problems identified, but also a few instances (like at least 1 of my issues) where the details are harder to pin down in a definite way.
The most obvious and easily tested one requires a relatively large library, when it gets into the range of 50+ (depending on screen resolution and zoom settings), scrolling through your library can cause very obvious visible flickering effects as the app re-loads already-loaded assets instead of just keeping them in memory while off-screen. This behaviour in several parts of the app is also responsible for memory leaks which can in some instances result in entire system crashes if you leave your PC running with Steam in the background for long enough. That's extremely rare, but the app eating upwards of 4GB of RAM is fairly common on PCs which are left to run for longer periods of time without restarting.
One of the (very) few issues they actually did fix was that many game icons were animated, and there wasn't any way to disable those animations. So they added the option. Except not really, because when they first added the option, it didn't work. So they "fixed" it. And it only took about 6 months of them "fixing" it repeatedly before it ended up actually getting fixed.