With a 5,400 rpm, just the slow rotation can introduce a wait time of 11 ms. Seek time from moving the arm can incur up to 20 ms (although some of that might overlap with the rotation time). And if it's a ten year old HDD, there might be a re-read necessary.

Even if the average read time would be just 20ms per file, cutting out ten of these reads saves 0.2 seconds, which is already noticeable to humans. And that's under the unrealistic assumption that Firefox is the only program trying to read from that poor slow HDD.

On a sidenote, laptops with 5,400 RPM drives are still sold.