If an item takes a lot of data:

SELECT itemid, num FROM trends 
WHERE clock>=unix_timestamp('2012-03-05 11:00:00') AND num > 120 ORDER BY num DESC LIMIT 100;
itemid num 
-------------------- ----------- 
11 116473 
11 54078 
11 10069 

Zabbix server can not process all this data by DBSyncers and Trappers can not insert data to history cache. In this case normal NVPS (3.5-4 Knvps) is down to 200-300 nvps.

It happens because history cache implementation: all new values should be processed by clock, so if one (several) items will have a lot of data per second, then Zabbix DBSyncers can not separate data for one item between a few DBSyncers, so one syncer will LOCK cache for full time, while other syncers will say that cache is empty.