Setting aside what 1.6x means, Rhino is much much slower than V8 or Java. In fact, there's a bit of an elephant in the room here because people don't really care about Nashorn vs. Rhino; they care about Nashorn vs. V8.
My sudoku solver http://dancing-links.herokuapp.com/ in javascript takes roughly a tenth of the time to solve a particular puzzle when running in node/V8 than in Rhino.
There is a big difference between interpreted rhino vs. compile rhino. The Rhino wikipedia page claims compiling to byte code "... produced the best performance, often beating the C implementation of JavaScript run with just-in-time". However, the benchmarks I've seen put Rhino compiled to byte code at about 60% slower than comparable Java code.