how often those functions get executed in three seconds

ldecay_speed_test.lua

local os_clock = minetest.get_us_time
local TIME = 3*1000000
local function time_sth(fct, ...)
	local start = os_clock()
	local fin = start
	local total = 0
	while fin-start < TIME do
		fct(...)

		total = total + 1
		fin = os_clock()
	end
	return total
end

minetest.register_node("ac:nodepar2speed", {
	description = "npar2speed",
	tiles = {"ac_block.png"},
	groups = {snappy=1,bendy=2,cracky=1},
	sounds = default.node_sound_stone_defaults(),
	after_place_node = function(pos)
		print(time_sth(function(pos)
			local node = minetest.get_node(pos)
			node.param2 = 1
			minetest.set_node(pos, node)
		end, pos))

		print(time_sth(function(pos)
			minetest.get_meta(pos):set_string("nodecay", "true")
		end, pos))

		print(time_sth(function(pos)
			local decay = minetest.get_node(pos).param2 == 1
		end, pos))

		print(time_sth(function(pos)
			local decay = minetest.get_meta(pos):get_string("nodecay") == "true"
		end, pos))
	end,
})

--[[
results:

361345
1624376
2590331
1805276

367324
1716379
2719357
1846280

297543
1444709
2156300
1621505
]]

see minetest/minetest_game#947 (comment)