You can just store a variable reference to the actor in begin play and reference in the tick.
BTW - GetAllActorsOfClass actually performs quite well - the engine keeps a HashTable for each class and uses that at runtime. I wrote a blog article on it: http://www.casualdistractiongames.com/single-post/2016/09/15/Inside-UE-Source-FUObjectHashTables-the-magic-behind-GetAllActorsWith