Shortest path from starting cell to all cells in the grid
You have an unweighted, undirected graph with $N$ vertices, and where each vertex has degree $K$.  Then the problem of computing shortest paths can be solved in $O(KN)$ time using simple breadth-first search.

Why is this faster than Floyd-Warshall etc.?  Because Floyd-Warshall has to handle weighted graphs.  In your case you have an unweighted graph: every edge has weight 1 (the length of a path is just the number of edges in it).  There are faster algorithms for computing shortest paths in unweighted graphs than in weighted graphs; unsurprisingly, the problem is harder if you have a weighted graph.  If you have an unweighted graph, you don't need the fancier algorithms, like Floyd-Warshall; breadth-first search is enough.