The engine. Ex employees, hundreds of em, wrote mostly about how horrific the engine and tools were when they worked at Bungie. One employee said just to move a pillar 2 metres he would have to leave the map loading overnight and pray nonstop that it didn't crash again by the next morning he got back in. The engine was made for Halo on original Xbox, before even 360. It is so outdated, old, slow, and overworked, it can't handle anything anymore. Hense Destiny Content Vault had to be introduced cause the engine was crashing more than it was loading so they had to gut the game. The game is slow at processing its own code and images. I wouldn't be shocked if Lightfall is the most unplayable glitchy -blam!- in Destiny history. You may ask "Why did they choose it then?". See when Activision had Bungie by the balls, in the contract, you can read for yourself, Activision forced Bungie to use their own engine and not pay other companies from sales to use their engines (like Unreal). This was still an obligation when Destiny 2 was created and Bungie had only one option, PULL OUT THE OLD AND DEAD HALO ENGINE FROM BEFORE 2004! Bungie is still using the engine and fighting with it cause everything was made in it so splitting with Activision didn't open the door for a new engine. Bungie had no time to create Destiny 1, a new engine from scratch AND redo the Destiny 1 story from scratch cause they lost key employees last minute back then. Bungie didn't think ahead cause of one of the words I just said "think".