It is possible to set Update interval for Zabbix agent (active) items to 0. Which causes agent to perform checks and send values to Server without delays.
In my example system.uname key sends 2168 values in 1 second.

This also works with Mass update.

Steps to reproduce:

1. Create/Edit any passive agent key
2. Set Update interval to 0
3. Set any custom scheduling
4. Update
5. Edit item and change type to Agent (active)
6. Update