The StyleManager::calculateCheckSum method calculates a checksum using a MessageDigest read from an InputStream obtained from the URL of the stylesheet. The problem is that it reads bytes from the stream one byte at a time without wrapping the InputStream in a BufferedStream:

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

It should either wrap the original stream in a BufferedInputStream or else read an array of, say 4K, bytes using the read(byte[]) method. The latter approach is used by the NativeLibLoader::cacheLibrary method.