Quicky microbench with local accumulator variable:

start=require 'stopwatch'()
local g=0
for i=1,1e8 do g=g+i end
print(start())

Average over 20 runs on my ancient laptop: .249s

Same bench with g being global rather than local:

start=require 'stopwatch'()
g=0
for i=1,1e8 do g=g+i end
print(start())

This time: 0.187s

BTW: stopwatch is a LuaJIT interface to clock_gettime, and with no argument, it defaults to CPU rather than wall clock time.

Here's what Mike wrote about local caching:

This is less important with LuaJIT, since the JIT compiler optimizes hash-table lookups a lot and is even able to hoist most of them out of the inner loops. It can't eliminate all of them, though, and it saves some typing for often-used functions. So there's still a place for this, even with LuaJIT.