They sound exactly the same, regardless of whether the speech is fast or slow.
It is impossible to make a consonant cluster of a 'd' followed by a 't'. So, unless we make a very long and unnatural pause between the two words, we are forced to combine the two consonants into one. The two merge to become a 't'.
NB Don't be tempted to make the typical Italian mistake of inserting a vowel in an attempt to separate the two words ( "declined -ah -to be"). Just drop the 'd', and it'll sound much more natural!