So, if you're making game within limited budget (like Civ5) you have to inevitable sacrifice modding at release.
That's irrelevant.
Civ 5 was less moddable on release than Civ 4.
For instance, Aheadatime says he had problems preventing stuff from spawning near city states.
In civ 4, the rules regarding placement of resources were defined in xml files. All map scripts could then refer to these and use the default values.
Civ 5 had everything defined in the map scripts. Sirian wrote a comment in the file trying to explain that was the lead designer's decision and it did have some advantages. Having coded a map script that eventually got included in official patches, I read Sirian's comment as a polite way to say "Jon made a poor design choice, I'm sorry about it, I had to live with it too".
The end result was that you simply couldn't do any modding of maps easily. Want to add a resource? Civ 4: Add it in the xml files.
Civ 5: Add it in the xml files, modify every map script that could use it.
The question is whether Civ 6 will be as moddable as 4 on release, or as 5 (or better or worse than either). Civ 5 was definitely worse at least as far as the map was concerned.
The performance issues are irrelevant. Lua is fast, and it's so much faster to code in than C++ that if you eventually reach a bottleneck you can refactor it into native code, but since in one month of lua coding you will have coded as much as you would have in 3 months of C++, you can better optimize and only optimize what is needed.