The WJEC Assembler is only supported for the 18M2.
Please note that this is not a true native assembler, it is a PICAXE emulation of an assembler which allows students to program the PICAXE as if it were being programmed in a native assembler language which is part of the EDUQAS/WJEC A Level curriculum.
The assembler languages supported by the PICAXE only have limited use outside the curriculum they are used within.
No idea as such, but it will be the usual "much slower" and probably "and then some".
The Assembler is converted to equivalent Basic commands behind the scenes, so "INC r0" might become "b23=b23+1" and that's what gets compiled, downloaded and executed. There can be multiple Basic commands for some Assembler instructions so it makes it difficult to put any figure on it.
For A-Level courses it's understanding the concept rather than execution speed which matters so it's not a problem for how it is. Students get taught flowcharting, high level language programming, and assembly language, so this allows schools to teach everything on the curriculum using a PICAXE which makes it easier and cheaper for them.