A DESCRIPTION OF THE PROBLEM :
com.sun.javafx.css.StyleManager.calculateCheckSum uses a raw InputStream as the input to a DigestInputStream. The InputStream should be wrapped in a BufferedInputStream to improve performance. Thus:

try (final InputStream stream = url.openStream();
       final BufferedInputStream bstream = new BufferedInputStream(stream);
                    final DigestInputStream dis = new DigestInputStream(bstream, MessageDigest.getInstance("MD5")); ) {
                    dis.getMessageDigest().reset();
                    while (dis.read() != -1) { /* empty loop body is intentional */ }
                    return dis.getMessageDigest().digest();
                }

Alternatively (preferably) move the IO to java.nio.file.