pt-archiver affecting online queries


When I execute pt-archiver it impacts my online queries. It causes them to go slow to a point where I have to stop pt-archiver process and restart mysql. Just stopping pt-archiver process doesnt stop my queries from returning to normal execution. They keep getting slower and slower even after I stop pt-archiver. This happens after some period of execution (of pt-archiver). It doesnt happen instantly after pt-archiver executes. It builds up over time and the queries tend to go slow after about 20 minutes of pt-archiver execution. I have noticed that the queries tend to get slow when pt-archiver is deleting the data not when it is selecting it. When I restart mysql the queries run normally in less than a second. Any thoughts on this will be very useful. Its a pain having to restart mysql every now and then.