I am working on implementing the suggestion of Alex to use structures to store the data for each fake cursor on a per window basis, instead of the prior method that created a Lisp_Object of cons cells (causing the excessive garbage collection issue).

It seemed to me that the implementation of creating the vector to store glyph rows looked somewhat similar to what we want to do here.  I attempted to borrow a portion of that vector glyph row implementation in the attached working example that applies to the master branch as of 04/08/2019 (a038df77de7b1aa2d73a6478493b8838b59e4982).

I had hoped to keep the cache (w->mc_elts) alive for the duration that the fake cursors feature is active in a visible window.  However, the cache (w->mc_elts) disappears and becomes NULL.  In the attached example, whenever it is time to increase the size of the dynamic vector, I set the value of w->mc_elts[99].x to 99.  The value of -1 is used in the stderr printout whenever w->mc_elts is NULL.

How can I make the cache (w->mc_elts) persistent without automatically becoming NULL?

In the attached example, I launch Emacs after applying the patch and evaluate:  (setq crosshairs t)

w->mc_nelts (100) | w->mc_elts_allocated (100) | w->mc_elts[99].x (99)
w->mc_nelts (200) | w->mc_elts_allocated (200) | w->mc_elts[99].x (99)
w->mc_nelts (300) | w->mc_elts_allocated (300) | w->mc_elts[99].x (99)
w->mc_nelts (400) | w->mc_elts_allocated (450) | w->mc_elts[99].x (99)
w->mc_nelts (500) | w->mc_elts_allocated (675) | w->mc_elts[99].x (99)
w->mc_nelts (600) | w->mc_elts_allocated (675) | w->mc_elts[99].x (-1)
w->mc_nelts (700) | w->mc_elts_allocated (1012) | w->mc_elts[99].x (99)
w->mc_nelts (800) | w->mc_elts_allocated (1012) | w->mc_elts[99].x (-1)
w->mc_nelts (900) | w->mc_elts_allocated (1012) | w->mc_elts[99].x (-1)
w->mc_nelts (1000) | w->mc_elts_allocated (1012) | w->mc_elts[99].x (-1)
w->mc_nelts (1100) | w->mc_elts_allocated (1518) | w->mc_elts[99].x (99)
w->mc_nelts (1200) | w->mc_elts_allocated (1518) | w->mc_elts[99].x (-1)
w->mc_nelts (1300) | w->mc_elts_allocated (1518) | w->mc_elts[99].x (-1)
w->mc_nelts (1400) | w->mc_elts_allocated (1518) | w->mc_elts[99].x (-1)
w->mc_nelts (1500) | w->mc_elts_allocated (1518) | w->mc_elts[99].x (-1)
w->mc_nelts (1600) | w->mc_elts_allocated (2277) | w->mc_elts[99].x (99)