OPcache ships with 5.5 and is off until you enable it

PHP 5.5 bundles Zend OPcache and does not enable it. A server that was running APC for its opcode cache and got upgraded in place therefore ends up with no cache at all — APC does not build against 5.5, and its replacement is sitting in the tree, disabled, while every request recompiles the application.

; php.ini — the extension ships with 5.5 but nothing loads it
zend_extension=opcache.so

opcache.memory_consumption=128
opcache.interned_strings_buffer=8
opcache.max_accelerated_files=8000
opcache.revalidate_freq=2
opcache.fast_shutdown=1

OPcache is an opcode cache and nothing else. APC also provided a user cache — apc_store(), apc_fetch() — and that half is not included, so an application that used both needs APCu alongside it rather than instead of it. max_accelerated_files is the setting that fails quietly: once the table is full, files are evicted and recompiled on every request while every status page still reports the cache as enabled, so read the wasted-memory and cached-scripts figures from opcache_get_status() instead of trusting phpinfo(). Setting opcache.validate_timestamps=0 is worth the last few percent, on the condition that the deploy script learns to reload PHP-FPM the same day you set it.