I did the same years ago. Ran 13.80's on street tires. Woo Hoo not bad for a street car in the early 70's When I added slicks at the track I lost over a second on my ET with the slicks, 14.80's I repeated the scenario several times at Fremont drag strip.

At 19 years old, I was Depressed and Totally baffled as to how I could go so much slower with slicks,
My mentor came to the rescue as I was FINALLY ready to listen to him. We redid the rear suspension aligning the springs and hangers, rebuilt the front end (which it needed).
The first pass on the same slicks netted a 12.65 with no other changes. Bolting the street tires back on netted a 13.20.
He explained to me that the rear suspension was binding due to the spring alignment, which in turn was further complicated by the sloppy front end.
I learned a lot from him and I hope he is smiling from above as I write this