SPJS 1.86 is live including a binary for Mac OSX. Let me know if you find any issues. Has best TinyG buffer, better serial port packet handling, and Arduino firmware programmer.
Correct. @Alden_Hart may have more to add to the status there. It’s somewhere between SPJS still needing to steer commands to a 2nd port or a new feature called “line mode” which is in this release.
The G2 seems fairly useless without a way to reliably stop it from executing commands without pulling the power, unless there is another GUI that I am unaware of anyway? @Alden_Hart can you chime in? I ended up picking up a ESS and Mach4 for my machine until this issue gets resolved, so I am not hurting as bad as I was before, but I would love to see this fixed so I can return to the fold 
I just ran the ChiliPeppr logo on G2 and it does run fine on 1.86 and stopping is fairly responsive. I have had sluggishness on G2 as well like you, but the current tinygg2 buffer on firmware 87.01 or higher isn’t unusable.
@Anthony_Webb We are working with @jlauer on a communications protocol that will address this but I’m afraid it’s not out yet. That’s one of the reasons we have g2 as experimental and have not yet released a board with g2. There are other UIs / products that are using g2, including the OtherMill, Shopbot Handibot, and the Printrbot CNC machines, but they have written their own interfaces. It’s high on the list to get the new protocol running with Chilipeppr soon. Thanks for the patience.
Just to clarify for the casual reader: G2 handles a feed hold command immediately, regardless of the buffered moves in the system. We also expose two serial ports to the computer so that there’s no problem with the OS buffering gcode data and preventing important commands like feedhold from getting through to the G2 board immediately.
The difficulty and problems John and Alden are working through is getting SPJS to reliably get an immediate signal (like feed hold) through with one serial port, while buffering enough data on the OS side in order to keep feeding moves to the TinyG fast enough that there are no pauses in the motion. The division of labor between the browser and SPJS makes using two serial ports difficult.
For the other UIs that don’t have that division of labor this isn’t a problem and has worked for some time.