[nailertn]

global_variable = 1

for i = 1, 1e8 do
	global_variable = global_variable + 1
end

On my PC this runs in 4.98 seconds.

local local_variable = 1

for i = 1, 1e8 do
	local_variable = local_variable + 1
end

And this in 0.96 seconds.

A very significant difference but I consider it clutter unless it's for something I use a lot, say for an OnUpdate or every CLEU / UNIT_AURA in a raid.