Tri top during races.

I am doing my first tri next week (sprint distance at Clemson,SC). The swim would be my strong suit & despite the low 70s temperature I have decided to forego the expense of a wet suit (I figure I will only be in the water for 10:30-11:00 min's). I bought a tri-top & swam in it yesterday for the first time. I am not sure if I had a bad swim or if the small pockets on the back of the top created more drag (100 yard time was off by about 5-10 secs!!). Are they actually made to wear under a wet suit? Should I rather save it for wearing under a wet suit? I will be trying it out on my regular swim this week & maybe will be able to answer my own question but would appreciate some advise from the more experienced pro's. I would like to wear it to keep my HRM on during the swim.