First game and first 3D game load fast but the others start up very slowly. I get the "prepare to enter the catacomb" screen really long. I thought dosbox had crashed or something but after a while the game starts up. The game itself runs without issues.

Any idea for a fix?

Also the run option doesn't seem to work.. I mean pressing shift to run in the game.

Since DosBox accurately emulates old computers, it also accurately emulates the same long loading times they had back in the 90's.

The only thing that might help is pressing Ctrl+F12 a few times to boost CPU cycles while it loads, and then Ctrl+F11 to lower back to normal when it finishes.

Also, the run button is Tab or V, not shift. I don't think the first 3D game had that feature yet, though. You could also use the DosBox Mapper(Ctrl+F1) to change the controls, too, including WASD movement with mouselook and shift to run.

Okay thanks!

You can try closing any other program that runs in the background including your anti-virus that runs while you play the game.