You might have spotted the checkbox next to the SNMP interfaces section, Use bulk requests:

When requesting values from SNMP hosts, Zabbix may request one value at a time or multiple values in one go. Getting multiple values in one go is more efficient, so this is what Zabbix will try to do by default—it will ask for more and more values in one connection against a device until all SNMP items can be queried in one go or the device fails to respond. This approach enables us to find the number of values that a device is configured to return, or is technically capable of returning, in one go. No more than 128 values will be requested in one attempt, however.

Only items with identical parameters on the same interface will be queried at the same time—for example, if the community or the port is different, Zabbix will not try to get such values in one attempt.

There are quite a lot of devices that do not work properly when multiple values are requested; hence, it is possible to disable this functionality per interface.