So there is a paper A Hybrid GPU Rendering Pipeline for Alias-Free Hard Shadows
That claims to calculate the distance to a triangle $d(\omega,T)$ efficiently, they
resort to some tricks based on the concepts of barycentric coordinates
they describe the squared distance between a point $w$ and some vertex $v_i$ of the triangle $T$ as
$d(\omega,v_i)^2=\left\Vert \omega-v_{i}\right\Vert=\lambda_{i-1}^{2}\left\Vert e_{i-1}\right\Vert ^{2}+\lambda_{i+1}^{2}\left\Vert e_{i+1}\right\Vert ^{2}-2\lambda_{i-1}\lambda_{i+1}\left(e_{i-1}\cdot e_{i}\right)$
There is a derivation in the paper.