abでベンチとってみた。
shop.example.comがMySQL、shop2.example.comがPostgreSQL
PostgreSQLの方が4倍くらい速い？

# ab -n 100 -c 10 http://shop.denden-cafe.net/products/detail.php?product_id=1

Concurrency Level: 10
Time taken for tests: 21.144408 seconds
Complete requests: 100
Failed requests: 0
Write errors: 0
Total transferred: 1434800 bytes
HTML transferred: 1398200 bytes
Requests per second: 4.73 [#/sec] (mean)
Time per request: 2114.441 [ms] (mean)
Time per request: 211.444 [ms] (mean, across all concurrent requests)
Transfer rate: 66.26 [Kbytes/sec] received

# ab -n 100 -c 10 http://shop2.denden-cafe.net/products/detail.php?product_id=1

Concurrency Level: 10
Time taken for tests: 5.869597 seconds
Complete requests: 100
Failed requests: 0
Write errors: 0
Total transferred: 1435000 bytes
HTML transferred: 1398400 bytes
Requests per second: 17.04 [#/sec] (mean)
Time per request: 586.960 [ms] (mean)
Time per request: 58.696 [ms] (mean, across all concurrent requests)
Transfer rate: 238.69 [Kbytes/sec] received