luacmd.lua

local func_tbl = lua.get_functions_table()
local set_lua = token.set_lua
local new_luafunction = luatexbase.new_luafunction
return function(name, func, ...)
  local idx = new_luafunction(name)
  set_lua(name, idx, ...)
  func_tbl[idx] = func
end

perf.txt

% Current implementation
8.44e-6 seconds (33 ops)
1.12e-5 seconds (43.8 ops)
% Current implementation (again)
8.92e-6 seconds (34.3 ops)
1.1e-5 seconds (42.8 ops)
% token.scan_string
6.82e-6 seconds (26.6 ops)
7.57e-6 seconds (29.3 ops)
% token.scan_string + luadef
1.82e-6 seconds (7.12 ops)
2.45e-6 seconds (9.55 ops)

sample.tex

\documentclass{article}
\usepackage{expl3,l3benchmark}
\begin{document}
\ExplSyntaxOn
\benchmark:n{
  \str_if_eq:eeTF{Some simple string}{Some simple string}{}{}
}
\benchmark:n{
  \str_if_eq:eeTF{\string\relax\csstring\ and enjoy your life}{\noexpand\relax and enjoy your life}{}{}
}
\cs_if_exist:NT \tex_luatexversion:D
   {
     \cs_set:Npn \__str_if_eq:nn #1#2
       {
          \lua_now:e
            {
              l3kernel.strcmp
                (
                  " \__str_escape:n {#1} " ,
                  " \__str_escape:n {#2} "
                )
            }
       }
   }
\benchmark:n{
  \str_if_eq:eeTF{Some simple string}{Some simple string}{}{}
}
\benchmark:n{
  \str_if_eq:eeTF{\string\relax\csstring\ and enjoy your life}{\noexpand\relax and enjoy your life}{}{}
}
\cs_if_exist:NT \tex_luatexversion:D
   {
     \cs_set:Npn \__str_if_eq:nn
       {
          \lua_now:e
            {
              l3kernel.strcmp
                (
                  token.scan_string(),
                  token.scan_string()
                )
            }
       }
   }
\benchmark:n{
  \str_if_eq:eeTF{Some simple string}{Some simple string}{}{}
}
\benchmark:n{
  \str_if_eq:eeTF{\string\relax\csstring\ and enjoy your life}{\noexpand\relax and enjoy your life}{}{}
}
\cs_if_exist:NT \tex_luatexversion:D
   {
     \lua_now:e
       {
         require'luacmd'('__str_if_eq:nn', function()
           l3kernel.strcmp
             (
               token.scan_string(),
               token.scan_string()
             )
         end)
       }
   }
\benchmark:n{
  \str_if_eq:eeTF{Some simple string}{Some simple string}{}{}
}
\benchmark:n{
  \str_if_eq:eeTF{\string\relax\csstring\ and enjoy your life}{\noexpand\relax and enjoy your life}{}{}
}
\ExplSyntaxOff
\end{document}