I play Dead Trigger 2 a lot when I feel like shooting up some zombies. There's no wait to play time other than for completing weapon upgrades and building (think consumables like health packs and grenades). Even then the wait is almost a non-issue because of how easy it can be to play. The only big downside is the storyline ends abruptly with no real resolution. You can keep playing infinitely, though, with side quests and tournaments.