Performance warning up front on the readme

While I assumed there would be a performance penalty I had no idea it was going to be 30x! I appreciate that you do have that in the docs but I think it's so important it should be basically the first thing on the github readme as a warning as I imagine it disqualifies this project from even being considered for a huge number of use cases and would prevent people from wasting time implementing before stumbling upon the performance docs while trying to figure out what they are doing wrong.