I also have noticed that people hold onto the chair/pew in front of them, hold their hands below their waists, etc… I also know that many of them (whom I know personally) are very devout Catholics. As for myself, I was taught very young (under 4 years old) to place my palms together at chest height when at Mass, during prayer, and especially during the Consecration and during the distribution of the Holy Eucharist or when receiving the Holy Eucharist. It’s a habit I’ve maintained for over 65 years now. The only ones who used to do the same during Mass were the Priest and Altar Servers. Now I’ve noticed a few other women doing the same, who can see me during the Mass. I don’t do this to “look holy” or to make any impression, it’s just how I was taught, and I would feel very awkward doing anything else. My husband (of blessed memory) was Protestant when we married, but had already decided to become Catholic prior to our marriage. He had always done the same (and got quite a bit of criticism for it in his Baptist Church!), but never stopped doing so. I notice that the men, more than women seem to not know what to do with their hands during the Mass. (wonder why?)
Doesn’t really bother me anymore, although due to my early training, it seems “disrespectful” to me, that’s my problem, not theirs!. I figure that the state of their soul is more important than the state of their hands! Be gentle, and forgiving of them. If they see you doing this (folding your hands), perhaps it will gradually spread through your congregation. I’ve noticed that people seem to imitate those they see around them at Mass. Here’s your opportunity to spread it around!