My favorite example of reducing gameplay challenge to speed up narrative pacing for a climax is Half-Life 2. Effectively, the game turns on cheat mode and asks you to complete basic gameplay challenges with this new wrapper until the game ends. What I found so interesting about this is the sense of "speed" and "fun" I had at game end as opposed to the "grind" and "frustration" via the small platforms and death pits of a typical end-level in Mario.
What happens is Gordon Freeman has all his equipment stripped away and replaces your most interesting item -- the gravity gun -- with a much more powerful version of the same item. Where before you could grab planks of wood and relatively minor items to complete gameplay challenges, now you could rip large panels out of walls and grab enemies, energizing them with a supercharged field of energy that destroyed everything that entered its throw-trajectory. It made dealing with enemies very easy, and puzzles were either new-but-easy or just super-sized versions of puzzles you had already mastered. But the novelty and sense of power kept me hooked gave me a thrilling antithesis to the usual end-game grind.
Some knock Half-Life 2 for a muddied and unsatisfactory ending -- I won't defend the quality of its climax -- but among its more-than-fair-share of amazing moments, it demonstrated that game design could make the final moments leading to climax thrilling rather than painful, and better mirror the pace of a dramatic arc.