You're sacrificing performance, not the end result. However, a lot of the performance tricks do carry over (such as object pooling for example.)

But for 99% of the web this doesn't matter at all. I'll take declarative over imperative any day if it means I can spend the saved time polishing the product instead of debugging or optimizing.

You have no idea the amount of debugging and optimizing which goes into developing a video game. Having worked on both I can say time spent debugging/optimizing is at least an order of magnitude more than for web applications. Its much worse when you're using an in-house engine as well.