From: Iaki Baz Castillo [mailto:ibc / aliax.net] 
# >> Benchmark.realtime { instance_eval 'a=123; a2=a*2; a3=a*3' }
# # => 4.50611114501953e-05

that's a string.

include a block, eg

> Benchmark.realtime { a=123; a2=a*2; a3=a*3 }
=> 1.59740447998047e-05

> Benchmark.realtime { instance_eval 'a=123; a2=a*2; a3=a*3' }
=> 7.58171081542969e-05

> Benchmark.realtime { instance_eval {a=123; a2=a*2; a3=a*3} }
=> 2.288818359375e-05