An example:
Zabbix web-interface allows to create trigger like this:

=0

When Zabbix server processes this trigger it tries to get ALL data before a clock:
12803:20130410:075811.300 query [txnlev:1] [select value from history_uint where itemid=23797 and clock<=1365559087]

So DBSyncers makes large overhead while processing trigger like this.