For Rails version 0.14 or greater you should upgrade to mysql-ruby 2.7 to get better performance.

For my application, the 2.6.7 version results in a 10-15% improvement for unchached pages:

perf data file 1: 10-29.all.trunk.mysql.2.5.1
  requests=1000, options=-bm=all -lib=trunk -mysql_session -fast_routes

perf data file 2: 10-29.all.trunk.mysql.2.7
  requests=1000, options=-bm=all -lib=trunk -mysql_session -fast_routes

page                               c1 real   c2 real    r/s    r/s   ms/r   ms/r  c1/c2
/empty/index                       1.48114   1.48793  675.2  672.1   1.48   1.49   1.00
/welcome/index                     1.63852   1.63111  610.3  613.1   1.64   1.63   1.00
/rezept/index                      1.49519   1.48949  668.8  671.4   1.50   1.49   1.00
/rezept/myknzlpzl                  1.48423   1.48344  673.8  674.1   1.48   1.48   1.00
/rezept/show/713                   4.10559   3.58523  243.6  278.9   4.11   3.59   1.15
/rezept/cat/Hauptspeise            5.01344   4.55163  199.5  219.7   5.01   4.55   1.10
/rezept/cat/Hauptspeise?page=5     5.11826   4.65202  195.4  215.0   5.12   4.65   1.10
/rezept/letter/G                   5.03240   4.58985  198.7  217.9   5.03   4.59   1.10