Around 2001, I ran the contemporary BeOS demo on a Pentium MMX 200 MHz machine with 32 MB of RAM. Even with those limitations, the thing screamed. I believe it was live CD you downloaded and burned.
I am absolutely not surprised it works well on Athlon 64.
Ahh, the memories! (266 MHz PII, 64 MB RAM ... maybe upgraded to 384 MB RAM by the time I was quad-booting Debian, Win2k, BeOS and QNX)
Maybe I ran BeOS slightly before a demo CD was available, or maybe I just didn't risk burning a coaster. (Remember those days where you had to worry about your OS not being able to feed the CD burner as fast as it was writing?) When I demoed BeOS around 2000, it was on a floppy (I repurposed a free AoL floppy from a few years earlier... by that time AoL was mailing free CDs instead of free floppies). The demo floppy allowed one to format a BeFS partition on the drive, and I think even put the kernel on the drive, but kept the bootloader on the floppy to encourage purchase.
I woke up one morning to see the floppy drive light on, and apparently a BeOS kernel or usespace driver bug caused it to spin the floppy continuously all night without moving the read/write head. I popped out the floppy and pulled back the dust guard to discover a thin stripe where the magnetic media had been polished off of the floppy. The drive didn't read any floppy correctly after that; presumably the read/write head was covered in magnetic media dust.
I don't remember how, but I eventually found instructions for copying the bootloader off of the downloaded floppy image and getting GRUB to find it, so I didn't need to put my replacement floppy drive at risk.
I remember having the exact same experience on slightly less powerful hardware with that BeOS demo. I remember throwing everything at it and it just kept on going like it was no big deal and me constantly going "wow, wow, wow" haha! It was such a bummer going back to Windows after experiencing that.
Pentium 75 mhz was enough for the BeOS demo. It was almost like using QNX. I believe I tried BeOS on some 486'es too, but if I did not at least it screamed, and burned, as you said, even on a Pentium 75mhz. The only limitation of the 'demo' was that usable space was like locked into 512MB extendable user-space, if I'm not wrong. Please do correct me/this.
BeOS never (officially) supported 486 class processors, I can't recall if it actually uses Pentium instructions and won't run, or if it's just super slow on 486. I think it is actually compiled for Pentium.
I am absolutely not surprised it works well on Athlon 64.