Someone on Reddit calculated how much time Leclerc lost behind backmarkers.
Colapinto: It's unfair to him. He didn't slow Leclerc in any way, Leclerc passed him on lap 49 just as Colapinto was coming out of the pits. At that moment a blue flag for Colapinto appeared on the panel, but Leclerc was already ahead of him.
Lawson: Leclerc caught Lawson on lap 56 and Lawson got the first blue flag on the straight in sector 5 and at that point Leclerc had a 3.2 second lead over Norris. Lawson went through 7 blue flags and Leclerc didn't pass him until sector 13 before turn 12. Because of this, Leclerc lost 0.4 seconds, the lead was 2.8 seconds.
At the same time, Norris passed Colapinto on the long straight at the start of lap 57 and reduced the lead to 2.3 seconds thanks to DRS.
Stroll: Leclerc caught Stroll in Turn 2 on Lap 58, Stroll got the first blue flag in Turn 3 and Leclerc's lead was 2.2 seconds. Stroll went through 9 blue flags and Leclerc passed him as well as Lawson before turn 12. The margin over Norris was 1.9 seconds.
Due to Stroll slowing, Norris caught him and on the exit of the last corner, Stroll had a blue flag for Norris as well. Norris passed him as well as Colapinto on the long straight and thanks to DRS was right behind Leclerc, who was now only 1.2 seconds ahead at the first corner.
Verdict: Leclerc was very unlucky for the backmarkers. Lawson and Stroll ignored 7 and 9 blue flags respectively, and Leclerc always overtook them before slow turns in the stadium.
On the other hand Norris was lucky as he had double DRS twice on the long straight and in sector 5 as well, as the two DRS have the same detection zone. He passed Colapinto and Stroll on the long straight, Lawson in sector 5. Leclerc didn't have a single DRS because of this, Norris had DRS up to 6 times (3 times on the first DRS, 3 times on the second DRS).
Credit to user AlphaArietis91.
Leclerc said he thinks he would've lost position to Norris eventually regardless of the lapped cars, and I'm inclined to agree - but even so, that doesn't mean we can't question why they are blatantly ignoring blue flags. To me, they're two separate issues.