I felt the same way on my first fixed gear commute, which was just last December. It felt slower, more of a workout, and that's a good thing. It checks off two boxes of what I wanted from it, and probably you also judging from what you came up with. Sitting up where you can see traffic better and more comfortably, rugged tires, convenient rack and who cares the weight, I don't think that speed was a particular concern when you built that bike. Your bike makes perfect sense for a commuter

I can tell you that SS/FG is a different style of riding, more work but then later it isn't. You have to get accustomed to the more varied cadence, standing more often, the different riding position. Or at least I did, and then at some point it wasn't really more of a workout. But still slower, most of the time, not that it matters in the larger scheme of things.