BrowserJSObjectLinker is lazily looking up netscape.javascript.JSObject, but unfortunately this means it's going through the superclass-traversing loop in canLinkType (which involves a potentially security-checking getClassLoader()) on every link request that reaches it. Adding a

System.err.println("Checking " + clazz.getName())

into the loop and running jjs prints:

$ ./jjs
Checking jdk.nashorn.internal.runtime.ScriptFunction
Checking jdk.nashorn.internal.runtime.ScriptObject
Checking java.lang.Object
jjs> print("x")
Checking jdk.nashorn.internal.objects.Global
Checking jdk.nashorn.internal.runtime.Scope
Checking jdk.nashorn.internal.runtime.ScriptObject
Checking java.lang.Object
x

and so on. It seems it's easier to just eagerly look up the class once.