bench_to_split_lines.rb

N=1_000
line = "a" * 200 + "\n"
body = line * 300
Benchmark.bm{|x|
  x.report("splitonly") {
    N.times{
      buf = body.dup
      lines = []
      while !buf.empty?
        lines << buf.slice!(0,201)
      end
    }
  }
  x.report("regexp") {
    N.times{
      buf = body.dup
      lines = []
      while line = buf.slice!(/.*?\n/m)
        lines << line
      end
    }
  }
  x.report("index") {
    N.times{
      buf = body.dup
      lines = []
      while idx = buf.index("\n")
        lines << buf.slice!(0, idx + 1)
      end
    }
  }
}

result

$ ruby -rbenchmark hoge.rb 
       user     system      total        real
splitonly  2.320000   1.280000   3.600000 (  3.618215)
regexp  4.490000   1.240000   5.730000 (  5.754846)
index  2.020000   0.690000   2.710000 (  2.711234)