Professor Jed Shugerman was quoted in a Vox article about special counsel Robert Mueller selecting a grand jury for the Trump–Russia investigation.
Mueller going to a grand jury is not a big deal by itself. The big deal is that he is impaneling a special grand jury devoted to the Russia/Trump investigation. Normally, a federal grand jury of as many as 23 people sits for a year or 18 months hearing all kinds of crimes: murders, white-collar crimes, etc. Mueller was using one of those general grand juries in Virginia for the Flynn investigation.
But now Mueller has impaneled a special grand jury in DC focused on Russia, and that commitment means that 1) Mueller expects this new grand jury to be doing a lot of special work, and 2) it will be reviewing classified material. A special grand jury devoted to a single investigation is very rare. Perhaps a clearer signal of Mueller’s case is his recent hire of Greg Andres, a former prosecutor specializing in foreign bribery and fraud. Andres would not leave his law firm if he did not expect to be very busy and very central to this case.
As for timing, compare Mueller to the Whitewater investigation of the Clintons. The first special counsel, Robert Fiske, took about five months to get to a grand jury. It took Mueller just a bit over two months. He may be moving faster than most people expected.