Slow rendering of tilemap

main.lua

tiles_path = "tiles.png"
tiles_horizontal = 4
tiles_vertical = 4
tiles_img = nil
tiles_quads = {}
tiles_size = 8

map_width = 100
map_height = 75
map_tiles = nil

function love.load()

    -- load image
    tiles_img = love.graphics.newImage(tiles_path)
    tiles_img:setFilter("nearest")

    -- generate quads lookup table
    local iw, ih = tiles_img:getDimensions()
    for x=1, tiles_horizontal do
        tiles_quads[x] = {}
        for y=1, tiles_vertical do
            local qx = tiles_size * (x - 1)
            local qy = tiles_size * (y - 1)
            tiles_quads[x][y] = love.graphics.newQuad(qx, qy, tiles_size,
                                                      tiles_size, iw, ih)
        end
    end

    -- generate tilemap
    map_tiles = {}
    for x=1, map_width do
        map_tiles[x] = {}
        for y=1, map_height do
            local tx = math.ceil(love.math.random() * 4)
            local ty = math.ceil(love.math.random() * 4)
            map_tiles[x][y] = {x=tx, y=ty}
        end
    end
end

function love.draw()
    love.window.setTitle("FPS: "..love.timer.getFPS())

    -- render tilemap
    for mx=1, map_width do
        for my=1, map_height do
            local tile = map_tiles[mx][my]
            local quad = tiles_quads[tile.x][tile.y]
            local sx = (mx - 1) * tiles_size
            local sy = (my - 1) * tiles_size
            love.graphics.draw(tiles_img, quad, sx, sy)
        end
    end
end