1) Get enough sleep. If I get less than 7hrs, I start to seriously slow down and get a bit stupid. Simple things like multiplying single digits in my head becomes quite difficult and requires a lot of focus.
2) The homework may take awhile. Some of my weekly assignments take more than 8hrs to do. In my opinion, high school didn't really require any brainpower. It was mostly mindless memorization without requiring understanding. College may be different - you can memorize everything you want, but if you don't know how to apply it proficiently, you'll be screwed on the tests. And just doing the homework may not be enough - if you can do the homework with your notes handy, then you'll get credit for the work, and a bit of practice. But you should also try some of the homework without your notes; make sure that you really know what you're doing.
Concerning your computer, maybe you can save a bit of time by relying more on your laptop, and leaving your PC in one place.
I too was a nearly straight-A student (until 11th or 12th grade, when I kind of started a downfall, lost a lot of motivation to do much of anything) without really trying all that hard. The worst part about the assignments was writing (writing just hurts my hands) or the time taken. They didn't really engage my mind though. It was just along for the ride.
College: It's still a lot of memorization, LOTS of memorization, but with a bit more application. No more standardized tests, so I don't have that wonderful crutch anymore. Standardized tests were great for me, especially vocabulary tests. I hardly remembered the definitions, but I remembered the phrasing. Most of the tests were matching or multiple choice, too, so I never really needed to learn the material.
In college, there's still a hell of a lot of crap to be memorized, but there's also procedures. In my Product Design class, we're doing spur gear design. It's a very lengthy process that isn't especially difficult. It amounts to reading simple charts and plugging numbers into basic algebraic equations, over and over and over again. About 5 pages later, you've got the size of your gears, stresses on the teeth, and some idea of what material and grade of gear and pinion to use. It's tedious as hell, but not especially difficult. But I still need to know how to do it reasonably quickly and accurately to get by on the tests.
In college I've got a 3.55GPA (cumulative) right now, courtesy of a few lower-level classes that I already got a taste of in high school. This semester is not so kind - it's mostly stuff that I have never seen before. And I just am finding myself less and less motivated to continue, so I devote less time to the homework. Right now I'm looking at a C average this semester, and the possibility of losing one of my grants.
College can be challenging, but I'm finding that the biggest (only?) challenge thus far is motivating myself to devote adequate time to schoolwork.