Religion victories in Civ are poorly telegraphed in general. You can easily look at the minimap and see that someone is conquering everything, and poking at a player’s borders will show you that they’re technologically advanced, but religion and culture victories tend to sneak up on people.
Well, there’s your problem. Civ 5 had a thing where research took more science points to complete the more cities you had. The ideal number of cities to own was five. If you had even a single city over that, even if science output was maxed out in all cities, it would take longer to research anything than for a player with only five cities.