Actually ... only the first version of Smalltalk was done in terms of the NOVA (and not using BCPL). The subsequent versions (Smalltalk-76 on) were done by making a custom virtual machine in the Alto's microcode that could run Smalltalk's byte codes efficiently.
The basic idea is that you can win if the microcode cycles are enough faster than the main memory cycles so that the emulations are always waiting on main memory. This was generally the case on the Alto and Dorado. Intel could have made the "Harvard" 1st level caches large enough to accommodate an emulator -- that would have made a big difference. (This was a moot point in the 80s)
The basic idea is that you can win if the microcode cycles are enough faster than the main memory cycles so that the emulations are always waiting on main memory. This was generally the case on the Alto and Dorado. Intel could have made the "Harvard" 1st level caches large enough to accommodate an emulator -- that would have made a big difference. (This was a moot point in the 80s)