OP, it depends upon the people involved, where they are in life, how old they are,etc, etc. Within one year, I had two entirely different reactions to two different guys I was dating when they said the “L” word. Both said it within three weeks of us starting to date. The first guy who said it, scared me to bits and it wasn’t long after that I ended it. The second guy that said the “L” word within such a short time frame of meeting each other and starting to date, I married and we’ve been together over 13 years now. We both knew we had met our future spouse after the first week of dating and we were engaged 5 months after first meeting each other.
There isn’t any rhyme or reason sometimes to “taking it slow”. I disagree with you though that it means people are waiting to become physical with each other. When I used the term when I was single, it meant the relationship had the potential to become more but the mutual trust was still being built due to different styles of communication between me and the person I was dating. With my husband, we clicked from the start, trust was built pretty quickly. We both progressed in deepening our relationship at the same pace because we were communicating at the same level and trusting being vulnerable with each other. We were both older, mid 20s, I think that made a difference as well, both knowing what we wanted for our futures. We both had discerned marriage was each of our individual vocations before we met each other.
I hope that helps.