If you’re not running them that often then they shouldn’t really be high-level functions. They won’t be automatically garbage collected. It’s probably more performant to just write function per call at that point or not use any outright function at all.
Because of its location it will take longer to access, but not anything that could ever be noticed. Everything in Lua is object based, a function’s memory cost exists within its individual existence on the stack as well as whatever occurs when it is called.
I’d also like to add that at a certain point you’re more or less writing a Lua package, and you might as well use a module instead of an explicit table.