To fix the problem of a block time of 24 seconds you need to add an additional parachain collator.
I just started two local rococo relay chain nodes and one parachain collator.
Then after Brunos suggestion I started another parachain collator and now the block time is consistently 12 seconds as expected.
To run two parachain collators you can follow the commands:
# Collator1
./target/release/polkadot-parachain --collator --alice --force-authoring --tmp --port 40335 --ws-port 9946 -- --execution wasm --chain ../polkadot/rococo-local-cfde.json --port 30335
# Collator2
./target/release/polkadot-parachain --collator --bob --force-authoring --tmp --port 40336 --ws-port 9947 -- --execution wasm --chain ../polkadot/rococo-local-cfde.json --port 30336
More on this here.