Most OS's today have their roots in the 80's and 90's. If a time machine brought the latest version of Win/OSX/Linux to a user from that era, they would be familiar with all the concepts. BeOS was one of the most modern OS's in the 90's, using preemptive multitasking, C++ (pre 98 standard), optimised to run on multiple CPU's. By extension, Haiku inherits a solid architecture not too different from modern OS's, and since from the start it was designed to scale well across multiple CPU's, it works very well on todays hardware.
The real reason Haiku feels faster is 2 fold - the AppServer creates a thread per application, and a thread per window, and is paired with another pair of threads private to app server. The communication between application and AppServer is via message ports, which feels more responsive (at the sacrifice of an additional queing/message delivery costs). In the end, it feels faster, but if you measured the response time from a mouse move/press until the application received it, it will actually be a few ns slower. But it feels more responsive, so I guess it's a win for the user perception.
The second reason Haiku seems faster is that it's very lean, with less bloat, less layers between the kernel and the application. This can be a good thing (speed) and a bad thing (features). Engineering is always a compromise, and at this point in the design features of Haiku are inline with the expectations of current users. I wouldn't be suprised to see that the other mainstream OS's come full circle, and drop the 'designed for beginners' features and bling, and revert to the a more traditional 'designed for power users' with less bling, making Haiku GUI modern again