The issue happens because the algorithm used to calculate the center of
the selection first needs to create a TransData array. In this array,
the code calculates the "mirrored" elements which can be quite slow in
dense meshes.
The solution is replace this slow algorithm used for calculating the pivot
point with the fast algorithm used to calculate the position of transform
gizmos.
Pull Request: https://projects.blender.org/blender/blender/pulls/107203
968ecf6f8b