2015 Spark not running well at all.

[Jackmanus]

Hey all, so heres the story. Two and a half weeks ago I installed VTS on the spark, a couple days after that (two weeks ago) I took her out for the first time this year and it was only going about 35 MPH with around 6200 RPM. I was thinking that maybe it was due to having half a tank of gas from last year and half a tank of new, fresh gas in it so I ran the heck out of it and eventually it gave me more power and started to go the regular 48 MPH with 8200 RPM. Today I took it back out to the lake to run the rest of the little remaining old fuel out of it and once again it was going about 38 with 6400 RPM. After running it close to empty it sounded kinda rough so I took it out, filled it with premium fuel for the first time ever and put it back in. When I put it back in it still sounded rough and this time would only go 6 MPH. I immediately took it out and went home. Once I got home I hooked it up to the hose and it was getting the regular 8200 RPM (give or take a few.) Any ideas whats up? I'm gonna have a dealership look at it soon. Sorry for long post.