You are running the PHP version where is_a() triggers autoloading. 
Good news is, you benefit even more from the cached autoloader than 
others.

#0  Horde_Autoloader_Cache->mapToPath(2011-05-08) called at 
[/var/www/pear/php/Horde/Autoloader.php:20]
#1  Horde_Autoloader->loadClass(2011-05-08)
#2  spl_autoload_call(2011-05-08)
#3  is_a(2011-05-08, Date) called at 
[/var/www/pear/php/Date/Holidays/Driver.php:880]
#4  Date_Holidays_Driver->_addHoliday(endOfWWar2, 2011-05-08, End of 
World War 2 in Germany) called at 
[/var/www/pear/php/Date/Holidays/Driver/Germany.php:205]
#5  Date_Holidays_Driver_Germany->_buildHolidays() called at 
[/var/www/pear/php/Date/Holidays/Driver.php:252]
#6  Date_Holidays_Driver->setYear(2011) called at 
[/var/www/pear/php/Date/Holidays.php:149]
#7  Date_Holidays->factory(Germany, 2011, de_DE) called at 
[/var/www/horde4/kronolith/lib/Driver/Holidays.php:72]
#8  Kronolith_Driver_Holidays->listEvents(Horde_Date Object ([] => 
2011,[] => 11,[] => 21,[] => 0,[] => 0,[] => 0,[] => Europe/Berlin,[] 
=> Y-m-d H:i:s,[] => Array ()), Horde_Date Object ([] => 2011,[] => 
11,[] => 28,[] => 0,[] => 0,[] => 0,[] => Europe/Berlin,[] => Y-m-d 
H:i:s,[] => Array ()), 1) called at 
[/var/www/horde4/kronolith/lib/Kronolith.php:489]
#9  Kronolith::listEvents(Horde_Date Object ([] => 2011,[] => 11,[] => 
21,[] => 0,[] => 0,[] => 0,[] => Europe/Berlin,[] => Y-m-d H:i:s,[] => 
Array ()), Horde_Date Object ([] => 2011,[] => 11,[] => 28,[] => 0,[] 
=> 0,[] => 0,[] => Europe/Berlin,[] => Y-m-d H:i:s,[] => Array ())) 
called at [/var/www/horde4/kronolith/lib/View/Week.php:73]
#10 Kronolith_View_Week->__construct(Horde_Date Object ([] => 2011,[] 
=> 11,[] => 23,[] => 11,[] => 59,[] => 3,[] => Europe/Berlin,[] => 
Y-m-d H:i:s,[] => Array ([W] => 47))) called at 
[/var/www/horde4/kronolith/lib/Kronolith.php:2994]
#11 Kronolith::getView(Week) called at [/var/www/horde4/kronolith/week.php:19]
#12 include(/var/www/horde4/kronolith/week.php) called at 
[/var/www/horde4/kronolith/index.php:27]


#0  Horde_Autoloader_Cache->mapToPath(2011-05-08) called at 
[/var/www/pear/php/Horde/Autoloader.php:20]
#1  Horde_Autoloader->loadClass(2011-05-08)
#2  spl_autoload_call(2011-05-08)
#3  is_a(2011-05-08, Date) called at [/var/www/pear/php/Date.php:460]
#4  Date->Date(2011-05-08) called at 
[/var/www/pear/php/Date/Holidays/Driver.php:881]
#5  Date_Holidays_Driver->_addHoliday(endOfWWar2, 2011-05-08, End of 
World War 2 in Germany) called at 
[/var/www/pear/php/Date/Holidays/Driver/Germany.php:205]
#6  Date_Holidays_Driver_Germany->_buildHolidays() called at 
[/var/www/pear/php/Date/Holidays/Driver.php:252]
#7  Date_Holidays_Driver->setYear(2011) called at 
[/var/www/pear/php/Date/Holidays.php:149]
#8  Date_Holidays->factory(Germany, 2011, de_DE) called at 
[/var/www/horde4/kronolith/lib/Driver/Holidays.php:72]
#9  Kronolith_Driver_Holidays->listEvents(Horde_Date Object ([] => 
2011,[] => 11,[] => 21,[] => 0,[] => 0,[] => 0,[] => Europe/Berlin,[] 
=> Y-m-d H:i:s,[] => Array ()), Horde_Date Object ([] => 2011,[] => 
11,[] => 28,[] => 0,[] => 0,[] => 0,[] => Europe/Berlin,[] => Y-m-d 
H:i:s,[] => Array ()), 1) called at 
[/var/www/horde4/kronolith/lib/Kronolith.php:489]
#10 Kronolith::listEvents(Horde_Date Object ([] => 2011,[] => 11,[] => 
21,[] => 0,[] => 0,[] => 0,[] => Europe/Berlin,[] => Y-m-d H:i:s,[] => 
Array ()), Horde_Date Object ([] => 2011,[] => 11,[] => 28,[] => 0,[] 
=> 0,[] => 0,[] => Europe/Berlin,[] => Y-m-d H:i:s,[] => Array ())) 
called at [/var/www/horde4/kronolith/lib/View/Week.php:73]
#11 Kronolith_View_Week->__construct(Horde_Date Object ([] => 2011,[] 
=> 11,[] => 23,[] => 11,[] => 59,[] => 3,[] => Europe/Berlin,[] => 
Y-m-d H:i:s,[] => Array ([W] => 47))) called at 
[/var/www/horde4/kronolith/lib/Kronolith.php:2994]
#12 Kronolith::getView(Week) called at [/var/www/horde4/kronolith/week.php:19]
#13 include(/var/www/horde4/kronolith/week.php) called at 
[/var/www/horde4/kronolith/index.php:27]

I don't see those here. You need to find out where those class 
requests are coming from.

i tried the new Autoloader_Cache with my configured xcache-backend 
wich increased the performance dramatically.
i looked in the xcache admin panel what information is stored in the 
variable and there i see a lot of date entries:


Kronolith_Event_Ical' => 
'/var/www/horde4/lib/../kronolith/lib/Event/Ical.php',
   'Horde_Icalendar_Exception' => 
'/var/www/pear/php/Horde/Icalendar/Exception.php',
   'Kronolith_Driver_Holidays' => 
'/var/www/horde4/lib/../kronolith/lib/Driver/Holidays.php',
   'Date_Holidays_Driver_Germany' => 
'/var/www/pear/php/Date/Holidays/Driver/Germany.php',
   '2011-01-01' => NULL,
   '2011-01-06' => NULL,
   '2011-02-02' => NULL,
   '2011-03-19' => NULL,
   '2011-03-25' => NULL,
   '2011-05-31' => NULL,
   '2011-06-24' => NULL,
   '2011-06-29' => NULL,

perhaps there is a bug in some application that tries to map dates to 
php files !?! or these shouldn't be stored in the autoloader cache, 
because so the cache is polluted with those entries