I had 72mb of RAM in my Q650, and even that is substantial overkill for A/UX. It was really quick and responsive on that machine; the MacOS emulation is based on System 7.0 and the UNIX underpinnings themselves are quite tiny and light by today's standards (reminiscent of Linux back in the 1.3/2.0 kernel days) so *for the software you can make run on it* I'd say A/UX runs better than MacOS 8.1 does on the same hardware.
If you skip the MacOS emulation and log into a plain X11 session it makes a passable X terminal if you have another UNIX machine on your network, but I *think* it only supports 256 color depth, so between that and only supporting X11R5 out of the box it's pretty hit and miss running "modern" software through it. Likewise the "seamless" MacX server that runs inside the System 7 sessions is a really fun toy, but it's fragile and pretty limited. Be sure to set your expectations appropriately.
If you skip the MacOS emulation and log into a plain X11 session it makes a passable X terminal if you have another UNIX machine on your network, but I *think* it only supports 256 color depth, so between that and only supporting X11R5 out of the box it's pretty hit and miss running "modern" software through it. Likewise the "seamless" MacX server that runs inside the System 7 sessions is a really fun toy, but it's fragile and pretty limited. Be sure to set your expectations appropriately.