See the help page.

Look at the open issues here.
If you think this is an unknown bug or new feature, open a New Issue.

Updates come as fast as CCP will allow.
This is supposed to be every 5 seconds, but for some fleet statistics it takes longer because CCP gives old obsolete data.
In general, it takes about:

• <5 seconds for fleet member changes (join/leave/move)
• ~30 seconds for fleet ship changes
• ~30 seconds for fleet location updates

NOTE: PELD combat data updates are sent immediately and are in real time.

Yes, this project is open source, just like PELD. View it here.

Only the bare minimum. Almost all data simply passes through the server from either a PELD client, or from ESI, to the FC. It is not stored.
To reiterate, the highly opsec things like PELD combat data, fleet location, fleet ship types, are NOT stored on the server.
What is stored:

• Public information like names, item IDs, etc.
• ESI Tokens (these are revoked when you log off)
• A list of fleet members (only character IDs) for an FC's fleet. This is required to verify that someone connecting through PELD does indeed belong to your fleet. Fleet data is automatically deleted after 24 hours.

If you don't trust any server that you don't control, you can spin up your own PELD-Fleet instance. Read more about that here.

• Flask (python webserver)
• React (javascript framework)
• MongoDB (nosql database)
• Nginx (load balancer, reverse proxy)
• Eventlet (websocket/concurrent network server)
• Redis (session caching)
• Docker (deployments/containerization)

Demogorgon Asmodeous. View my github here. ISK donations welcome.