No. (At least, I wouldn't think so, since it's not an issue for one to use the line for US citizens and the other the line for Canadian citizens; and it's not an issue for one to use the NEXUS line and one the normal line.)

We went through GE at about 3:30pm on Tuesday (Sept. 20), and no one in the Nexus or MPC lines (and only 1 ahead of us in Nexus security - I wasn’t paying too much attention but don’t recall seeing many people even in the regular lines). We walked up to GE booth after using the kiosks (which couldn’t recognize my 4yo), officer already knew that last part and sent us to MPC booth to have her processed - line was empty so we walked right up. All in all, probably about 5 minutes to clear both security and immigration, and that includes having my 7yo backpack inspected (a pair of small art scissors were in there, and apparently they can’t tell they’re small on the screen). All in all, pretty easy process this time.

Data point for T1

Made it through US preclearance and catch my flight in a 1.5hr connection in the morning rush hour.

Wouldn't have been remotely possible without MPC/Nexus
YYZ is such a huge mess. The queue is backed all the way up past the checked luggage holding hall.
no signage no direction no instructions.

There was not a single person in the MPC line until I made the call and let everyone know that there are 3 seperate lines past the bottle neck. Everyone is too polite to question or investigate.
I made it through right away

The regular is solid, not moving AT ALL the whole time I was waiting for my checked bag to clear.

There are no priority line for Security Screening but it went through within 10 mins.

Around 8 am on a Friday at T3 - security was quick, 5 minutes to get through. US customs line was very long, I suspect over an hour. MPC line had maybe 5-10 people in it... I had an issue submitting my form as it kept telling me I’m too far away, even when I was standing in the customs hall. After a few tries it went through, and I was at my gate in no time.

I’m assuming once everyone learns about MPC, it’s not going to make a big difference, but right now it’s great. I suppose we should try to keep it quiet 🙂

Submit MPC no later than CATSA, connection is bad once you are in US preclear post security.

Data point from recent YYZ-LGA trip, Canadian passport. Departed YYZ T1 mid-morning, maybe 10 people in Nexus security lane, breezed through in a few minutes. Only one other person using Nexus machines at US customs, again straight through in no time flat. Flight delayed, but that's another matter.

Return the following day late afternoon, YYZ T1 customs hall was an absolute circus, thankfully only 6 - 8 people using Nexus. Love the new machines that I don't have to squint into, was back in my car within 10 minutes. Main customs line looked to be at least 45 minutes or more. Baggage hall utter chaos, so gad for carry-on only.

I'm not a frequent traveler. I'll be traveling YQB - YYZ - SFO - MEL (Australia). Can MPC help me? If so, I would be filling the form before my flight from YYZ and then use the QR code in SFO?

no - you pre-clear customs at YYZ. You’ll land at SFO just like any domestic US flight.

I haven’t connected to the US via YYZ in a long time (as it was more typical to originate there), but beleive it should be a separate queue from YYZ-originating passengers. I doubt there is a separate MPC line for connecting pax, but I’m not sure. Am 100% sure you’d clear in YYZ, not SFO though. Hopefully, you have good connection time for this.

The good news is once you land, process is much easier in SFO - in fact, I’m assuming you’d be flying AC or UA into SFO, then UA to MEL, so you’d land in terminal 2 (AC) or 3 (UA), and there are airside walkways now between them, as well as between 3 and G. Walk from T2 to G might be 15-20 minutes, but you wouldn’t have to clear security again.

There is different incoming transit passengers to US procedure depending on which terminal you arrive at YYZ as well as whether there is a terminal change required.

MPC would help to shorten the lineup. You will be using that to pre-release by US immigration at YYZ.

As a nexus card holder, is there any benefit in downloading the MPC app?

No there isnt. NEXUS is usually faster at US pre-clearance along with US ports of entry due to Global Entry

not sure what it would be. As MPC becomes more well-known, those lines are going to be longer.

On that note, in Sept, leaving YYZ with the kids, the kiosk somehow didn’t recognize use the 4yo. When we walked up to the booth, the officer sent us over to MPC officer to have her cleared, though didn’t really matter - both lines were empty when we came through.

Did YYZ T1 get rid of waiting for checked luggage on domestic to USA transfers? Or just there was a technical problem when I gone through there this Wednesday. The BP scanners were off and covered with messages Proceed directly to CBP.

Hello! I am flying YUL-YYZ-BNA. It shows that my AC flight from YVR to YYZ is coming in at T1 gate D34 and outbound AC flight from YYZ to BNA is T1 gate F34. I have Global Entry. I'd appreciate any insight into what I should expect in terms of Immigration/Security at YYZ. I will have only carry-on bags.

Walk the long, long walk from the D gates to the F gates, via the upstairs transfer corridors accessed by escalator/elevator near the AC customer service desk. There will be people walking in the opposite direction as you as they look for their D gate connections (they're new arrivals to Canada) or E gate departures (they just cleared security)

Following signs for F gates (with a US flag), you'll go through a dedicated transit security lane prior to US Preclearance. It's rare, but not impossible, for them to set up a Priority lane, so you might have to be lumped in with the large crowd. It's excruciating in the early mornings

You'll experience Global Entry 2.0. The kiosk would be expecting you based on previous pictures of your face on their database (and because you're flying today), you'll snap a new image, and if Preclearance is sufficiently busy, you'll be waved through by an officer after waiting about 3-15 seconds for them to compare today's image to previous images on their tablet computer. You'll make verbal custom declarations. There's a very low chance you'll need to whip out your passport after boarding your YUL-YYZ flight unless CBP needs to ask you some more questions about your stay in the US

And then you're through. My personal best time is about eleven seconds between leaving the GE kiosk and being "welcomed" into the United States, most of that time was walking up to the CBP officer with the tablet