Number 1 reason why there are delays in your U account is because its getting its fills from an F account. This is an IB limitation with F accounts.
Since we do not receive fill notifications for F accounts there will be delays spanning from seconds to minutes.
Subscribers will fill according to what the strategy shows.
With non-allocated IB accounts (U accounts not linked to an F account), we receive fills in real time and the signal is sent to C2 immediately.