Road tool with pathfinding performance issues


I designed a road tool that uses an A* pathfinding algorithm on a simple square-based grid - nothing fancy, as simple as possible.
Since it’s a road tool, I want it to work well on massive grid sizes. I’ve tried optimizing the grids, both data actor-based and struct array-based grids, but one’s working worse than the other: The “GetNeighbours” function which I need for pathfinding is killing performance.
Question is: Is it feasible to work with pathfinding on very large grids (128x128 or even higher) without loosing performance OR making it in C++ ? Or I’ve just coded it bad. (I have no programming knowledge/experience or talent for that fact, whatsoever)
The alternative would be to fake it, I just can’t figure out how, without adding ISMs/tick which looks/feels unsatisfying.

Thank you!