为什么“注入”很慢?

这是基准

require 'benchmark'

# create random array
arr = 40000.times.map { rand(100000).to_s }

r1 = ''
r2 = ''
r3 = ''

Benchmark.bm do |x|
    x.report {
        r1 = (arr.map { |s|
            "[#{s}]"
        }).join
    }

    x.report {
        r2 = arr.inject('') { |memo, s|
            memo + "[#{s}]"
        }
    }

    x.report {
        r3 = ''
        arr.each { |s|
            r3 << "[#{s}]"
        }
    }
end

# confirm result is same
puts r1 == r2
puts r2 == r3

这是结果

       user     system      total        real
   0.047000   0.000000   0.047000 (  0.046875)
   5.031000   0.844000   5.875000 (  5.875000)
   0.031000   0.000000   0.031000 (  0.031250)
true
true

有什么办法 inject 更快?