question on Makefile first flavor assignment

Hi gnu make friend,

I am trying to understand the first flavor of variable - recursively
expanded variable assignment disadvantages mentioned in gnu make manual,

Another disadvantage is that any functions (see Functions for Transforming
Text <https://www.gnu.org/software/make/manual/html_node/Functions.html>)
referenced in the definition will be executed every time the variable is
expanded. This makes make run slower; worse, it causes the wildcard and
shell functions to give unpredictable results because you cannot easily
control when they are called, or even how many times.

For the recursively expanded variable assignment  on Transforming Text
functions, it will be executed every time the variable is expanded, which
will cause make run slower.
Can you give a simple example to make it a little bit more clearer?

Thanks,
Tom