Asked this on answers.microsoft.com and was referred here:

LAMBDA functions and the assorted helpers make it possible to approach problems in a variety of ways, however I'm not aware of any documentation about measuring efficiency of these. From experimenting since the beginning of the year, I have developed intuition about what is inefficient because it will make Excel calculation refresh slow considerably, or crash the instance completely. I don't know how to preference one implementation over another as being "efficient enough" in case I might use some high-powered LAMBDA construction repeatedly in the same workbook. A concrete example I've used a few times is something like:

TimeIt := LAMBDA(fn,param,

LET(

start,NOW(),

execute,fn(param),

end,NOW(),

elapsed,end-start,

elapsed)

)

But this is dependent on a lot of variables, and not very precise.

How can LAMBDA efficiency be measured?