For improved functionality, a template was invoking a new module, Module:Code de liste décalable, but I had to remove this module invocation, after I noticed in the "NewPP report" that it increased the template execution time from 0.3 ms to 3 ms, approximately. As the template is usually used many times, and in most of the articles, this resulted in a noticeably increased rendering time.

Though, the Lua code itself is very fast. In most cases, all it does is a plaintext string.find(). Even if the input string may be (moderately) large, on my local machine the Lua code takes something like 1 µs.

For comparison, we have some frequently used templates on frwiki, {{Date}} and {{Unité}}, that have an underlying module (Module:Date and Module:Unité respectively), and although they are much more complex, I'm considering that every inclusion of these templates takes about 1 ms, as a rule of thumb. 80% of that millisecond being the invocation setup overhead, and 20% the actual Lua code (see T357199, on this matter).

Thus, I don't understand why the module I mentionned at the beginning, although it has an expectedly fast code, takes as much as 3 ms…