Sorry for the late reply, but yes, I believe it is, it's just arguably harder than having 2 players play!

For those interested, we have a discord server to discuss speedrunning of any of the BoxBoy games. You can join here: https://discord.gg/csYCBW4

The fastest time is even faster though. (ie. Ninja WR is ~10s, wheras the Qbby one is ~16s) I know when I first played this game my times were quite a lot slower. It was only once I had played the game a lot more that I was able to drop my times down a bit. There are plenty of techniques you can do to make yourself get faster times. If you watch any of the videos that are up (eg. http://www.speedrun.com/run/wzp4dovz) you can get some of an idea. For the most part it is down to doing the level quite a few times and really knowing the route.