I don’t think it really matters. In かわいい, there are two いs so the sound is extended. In かわいくなかった, there is only one い so I don’t see why the sound would be extended. But if you’re talking about mashing the わ and い together, I don’t believe the sound would be made super quick in English, but it wouldn’t be so stressed that it would return to the sound of two いs.
Syllables aren’t exactly mashed together quickly in Japanese; if you listen to Japanese songs, very often each hiragana character is said separately.