In theory APFS should be better at almost everything according to Apple
It's not better at sustained bandwidth, and that is known fairly well. Even HFS+ manages to outperform it in synthetic benchmarks. Not that it means anything in real world.
but the fact is that, for example, the boot time is much longer since High Sierra where they changed the file system.
Sorry, but who cares about boot times? These machines are designed to be always on. And the 30 seconds of boot when you have to surely won't kill you.
Besides, who says that the regression in boot time you observe is due to APFS? There are a lot of things happening during boot, any any of them could affect the overall time. For example, folks like to compare the boot time of Windows and macOS (Windows is undeniably faster), but they disregard the fact that macOS shows you the desktop after all services and standard apps have launched while Windows first shows you the desktop and then starts launching stuff. If you time the process until you get a fully usable, completely load system, Windows takes almost twice as long...
Since the time macOS uses APFS I haven't seen a single benefit of it in my use.
Sure you did (provided you use an SSD of course). More effective disk space usage, faster indexing, better security, more reliable backups, more responsive filesystem. HFS+ was a ticking time bomb.