Most efficient way to determine if a Lua table is empty (contains no entries)?

Your code is efficient but wrong. (Consider {[false]=0}.) The correct code is

if next(myTable) == nil then
   -- myTable is empty
end

For maximum efficiency you’ll want to bind next to a local variable, e.g.,

...
local next = next 
...
... if next(...) ...

(When next is local, the code finds primitive function next by a constant-time indexing operation into an array of “upvalues.” When next is left global, finding next involves indexing index the “environment” hash table, which contains the values of the global variables. This indexing operation is still constant-time, but it is significantly slower than the array lookup for a local variable.)