Slow variable refreash

Dear all,
I’m trying to do a receiver, i have to match te receiving signal with a
sequence created connecting some variable to a stream block, in this way
I have something similar a vector but now I can refresh the value of the
variable with self.const_source_0.set_offset(sequenza_PN[0]) because i
need to shift it to syncronize the signal.
The problem is that when i change the value, it take like 6 second
before have the real refresh with the correct output match.
Somebody can tell me the reason for this?

Thank you