bug#15294: 24.3.50; js2-mode parser is several times slower in lexical-binding mode

[Drew Adams]

> > Or is the byte compiler likely to be improved in this regard, so users
> > need not be aware of it and take measures manually?
> 
> As indicated in the FIXMEs I added in src/bytecode.c at the time, the
> current byte codes for unwind-protect, condition-case, and catch are
> very inefficient for lexical-binding code.
> 
> I do hope to fix those issues by introducing other byte-codes which will
> let us generate significantly more efficient code for those constructs,
> but in 24.1, the priority was to get lexical-binding to work correctly,
> performance being a secondary concern (tho for most idiomatic Elisp
> code, the performance tends to be competitive).
> 
> What people should know is that
> 
>    (let (x y z)
>      ...(setq x ...)
>      ...(setq z ...)
>      ...(setq y ...)
> 
> is often a bad idea in Elisp, and even more so in lexical-binding code
> (in some cases, if a variable is immutable it can be handled
> significantly more efficiently, so the mere existence of a single `setq'
> on a variable can sometimes slow other chunks of code: in many cases
> `let' is cheaper than `setq').

Thank you for both answers.  Will the second apply even after making the
byte-compiler improvements described for the first?  (I'm guessing yes.)
If so, you might want to mention this guideline in the manual.