You're slacking off is actually a viable (and common) survival strategy. Work too hard and they'd begin to expect 110% all the time. This way, when the real crunch-time occurs, you can just say "ok, gotta cut back on the Minesweeper for a bit" and they'll think you're busting your gut.
There a probably a whole lot of people only working "just hard enough" for their set paycheck out there.
Plus if what I hear so often is true, and most workplaces are just like the one I'm sitting in right now, then they don't provide you any incentive to work hard. They just take advantage of it when you do, and punish you when you fall too far below the average.
I'd like to think where I work is slightly different, but I know full well it's not that different. I'm salaried, and because my boss chooses to work sixty hours a week he seems to think everyone else has it in them to do so as well, and that "paid the same no matter the hours" means "you're an employee 24/7, the 15ish hours a day off are a bonus not a right". I've worked twelve hour days fueled by spite, I've worked on Saturdays for nothing more than a stern conversation, I've answered phone calls at midnight to talk through an issue until three in the morning.
I'll admit, my Christmas bonus of an extra month's pay is a pretty good mark that this company does actually give a shit about hard work, but there is a certain amount of "don't stick your neck out" born into me, especially thanks to my last job. And to be perfectly honest, I'm not sure that busting ass all day every day would actually be more productive. My boss has straight up said I've earned to right to put less time a day than other employees, because he sees me routinely get more done in eight hours than most people he's worked with do in nine or ten. Part of this is precisely because I slack off once in a while, I can work damn hard when I work but I factor a certain amount of decompression-time into everything I do.
It's easy to get addicted to decompression sure, but when people are actually counting on me to get something done, I actually produce results.