I upgraded my ruby 1.8 to the latest patchlevel and all of a sudden ruby-marc was super-slow. I found the same thing on 1.9 and in JRuby, so I investigated.
There's a marc.bytes.to_a call inside the loop in Reader#decode. All the fix does is move it outside the loop so it only happens once.
You can see the patch in the "slowreadfix" branch at https://github.com/ruby-marc/ruby-marc/commit/beba83745ebe0848218496e967edd65d632fb01e
As you can see, the speedup is about a factor of five.
Test case is reading in a Marc21 file with about 18K records in it.
user system total real ruby 1.8.7 (2011-06-30 patchlevel 352) [i686-darwin11.2.0] Strict (stock) 165.290000 1.480000 166.770000 (166.805960) Strict (fix) 25.190000 0.150000 25.340000 ( 25.333554) Forgiving (stock) 14.810000 0.080000 14.890000 ( 14.880064) Forgiving (fix) 14.540000 0.060000 14.600000 ( 14.611334) ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin11.2.0] Strict (stock ) 50.020000 1.260000 51.280000 ( 51.284811) Strict (fix) 9.530000 0.080000 9.610000 ( 9.608464) Forgiving (stock) 6.080000 0.020000 6.100000 ( 6.112313) Forgiving (fix) 6.320000 0.020000 6.340000 ( 6.345729) jruby 1.6.5.1 (ruby-1.8.7-p330) (2011-12-27 1bf37c2) (OpenJDK 64-Bit Server VM 1.7.0-u2-b21) [darwin-amd64-java] Strict (stock) 26.689000 0.000000 26.689000 ( 26.690000) Strict (fix) 5.061000 0.000000 5.061000 ( 5.061000) Forgiving (stock) 5.389000 0.000000 5.389000 ( 5.238000) Forgiving (fix) 5.092000 0.000000 5.092000 ( 5.054000) jruby 1.6.5.1 (ruby-1.9.2-p136) (2011-12-27 1bf37c2) (OpenJDK 64-Bit Server VM 1.7.0-u2-b21) [darwin-amd64-java] Strict (stock) 26.913000 0.000000 26.913000 ( 26.913000) Strict (fix) 5.336000 0.000000 5.336000 ( 5.337000) Forgiving (stock) 5.385000 0.000000 5.385000 ( 5.385000) Forgiving (fix) 5.219000 0.000000 5.219000 ( 5.219000)