I now do all of my back and forth via BUF and have encountered zero issues there over the past few months. The drive to Toronto is quicker than being imprisoned upon landing at YYZ. Plus no southbound covid test required. And cheaper airport parking.

Like WesternCDN's post above, the arrival debacle was no better at T3 last night (May 24, 2022).
Travelled AA1111 from DFW to YYZ. Landed at 4:30pm and at gate at 4:40pm. Delayed deplaning.

First announcement from pilot was "Canada customs won't permit deplaning. No estimate on wait time."
Next announcement from gate agent... "Canada customs have implemented a "customs metering program" and will advise when passengers can deplane. No estimate on timing".

Then she comes back on 2-3 minutes later to say wheel chair passengers and those with connecting flights will be permitted to deplane now. She reads off list of passengers who are in one of these categories. Whoo hoo... some common sense to let connecting passengers off! And I'm lucky since I'm taking my 90 year old mom back home and she's in a wheel chair.

Off we go and the attendant -- who is just marvelous -- whizzes us through the halls into T3 customs hall. So many people! We're moving so quickly with the attendant I didn't get chance to take any photos. But the regular Kiosks area looked completely full of people and even the NEXUS kiosks were all full. The attendant wheeled us right to the special services boarder officer (one also used by flight crews). Through without any questions (Both my mom and I are NEXUS, but didn't use, just used Canadian passport with officer).

Key Message -- Arrival deplaning delays not getting any better at T3. But perhaps some common sense setting in now as will permit connecting passengers to deplane first. Otherwise plan on up to an hour or more delay before deplaning after arrival especially around busy arrival times (late afternoon for example).
Update/Added info: Time of day and day of week likely has impact. For example, flight attendant I talked to today who regularly does DFW-YYZ-DFW (AA1993) said haven't seen issues. Likely due to arrival/departure time (lands YYZ at 11am / departs 12noon).

Arrived at YYZ 11a this morning (from ATL) at T3. No delays thru customs, just regular wait times.

I arrived from LHR at YYZ T1 around 2 PM on holiday Monday this week. Before landing the CC warned over the PA that customs metering could apply to our flight and then confirmed it once on the ground. A further announcement was made 2-3 minutes later that connecting passengers would be allowed to deplane and you had to show your onward boarding pass at the aircraft door to be permitted exit. Thankfully with ITD OSS, although there was only one Nexus machine working in the little transfer CBSA room I waited in the regular line for a few minutes and was in the awful MLL shortly after. No idea how long the poor saps with destination YYZ had to sit on board.

what are the chances of getting selected for random testing at YYZ?

No one knows. Definitively less than 50%. Probably less than 5%. But no one really knows.

For what it's worth, it looks like randomized testing is stopping (either immediately or soon) for passengers connected from an international flight to a domestic one.

No one knows ??
There is data on the government website, showing total number of arrivals and total number of tests, for each week. Latest numbers are 450K arrivals and 25K tests, so random sampling rate of arriving travellers by air is 5.5%.

My personal experience arriving from an international flight at T3 Friday afternoon.

AF356 touched down at 16:09. Was at gate maybe 10 mins later. Crew made announcement of delayed off boarding, as expected, with connecting passengers first, followed by business (which I was in). Connecting px were able to disembark right away, followed by business maybe 5 mins later. After the very long walk to the arrivals hall, it was very crowded, but I made the immediate right to Nexus where plenty of machines were available. Was in the baggage claim area at 16:48. Waited about 15 mins for luggage. Was outside at 17:10. So roughly an hour from touchdown to being outside. Not bad considering the stories from the media.

How do YOU know that “no one knows” ? At most you can say that YOU don’t know. Why presume that no one else knows either, and then spread that misinformation as fact ?

The fact IS that the data about arrival testing is published by the government and is available publicly (see link below). The data shows that in recent weeks the number of travellers arriving to Canada on international flights is about 450k each week, and 25k of them are “randomly” selected for arrival testing (each week). So there is currently a 5.5% chance of being selected, although I would hope and expect that this will be reduced in the future.

https://www.canada.ca/en/public-heal...llers.html#a42

May 22-28: YYZ-RDU-YYZ-FRA-BCN-YYZ

YYZ-RDU: Saturday around noon, transborder check-in and security quick and easy. Nexus line - one person ahead of me. Nexus line was just a regular security line, so shoes off, liquids out - the works. The officer mentioned that the regular Nexus lines are slowly being brought back. US customs understaffed, so a bit of a wait for Nexus customers - no longer than 10 min. YYZ-RDU delayed by 1.5 hours due to blah-blah-blame Pearson.

RDU-YYZ: Wednesday afternoon. US pre-check quick and easy. Flight delayed by 1.5 hours due to blah-blah-inbound-blame Pearson. Upon landing, connecting passengers let off first, within 5 minutes of gate arrival. Connecting to International took no time at all - everything is automated - no line, just a long walk.

YYZ-FRA: delayed by 1 hour due to blah-blah-blame Pearson. Bus gate at FRA. Two buses to get to FRA Terminal A. Looks like FRA is doing a major reno.

FRA-BCN: Delayed by 30 min, but no Pearson to blame.

BCN-YYZ: Delayed by 1 hour due to - you guessed it - inbound-blame Pearson. Arrival at YYZ on time. Connecting passengers off first - almost immediately. The rest of us in the front - in the next 10 minutes. Arrivals hall not super-busy. Got through Nexus in no time.

Overall, not too bad given the nightmarish scenarios from earlier this month. Hope things will improve even further.

I arrived yesterday from Munich on Air Canada at approx 2.30pm. Flight arrived early, waited 20 mins or so for a gate. Then they said there would be metering so only passengers with connections who could show boarding passes could leave. Which was more than half the plane. That took about 15 mins. Then they said 100 people could leave so I got to get off. Customs/Immigration hall was not busy at all so no idea why they were metering. Got selected for random Covid test which took less than 5 mins.
If they are going to meter passengers off planes, they should let Nexus passengers off because there is no line for Nexus, we use the automated kiosk and all the immigration staff are doing is putting a green or pink sticker for Covid screening.
And please check before implementing metering - if there is no lineup what is the point of metering.

Proof of a connecting flight
 

When you show your BP for the connecting flight, is it checked visually or scanned?

It was just a visual inspection by the FA (AC flight from Europe)

Might I suggest to all Nexus card holders to contact GTAA or your local airport re the current metering process to get off planes due to backlog at CBSA. I have been held on planes due to metering several times only to arrive at Nexus kiosks to find no lineup. As we know, with the new Nexus automated kiosks/gates, you can get out in 30 secs. The only thing the CBSA is doing is adding a green or pink sticker for Covid screening. So send your suggestions to your local airport (Contact us on their websites) that Nexus card holders should not be held for metering. Nexus card holders do not use the long lines for regular travelers including for connections or final destinations.
The point of having Nexus is to get prescreened and speed up the border process while the current airport process that includes Nexus card holders in the metering process is counter intuitive.

Trust me, they know. They just don’t care. Because they’re the most useless airport authority in the world. YVR is better, even MCO is better. And let’s not even bring up European airports, which is what they should be modeling. If this were to happen in ZRH, they’d hold a national referendum over firing the airport execs.