users/{id}/authentication/methods generates many 429 errors.

Hi,

In a large inventory script one of the lists it generates contains all registered MFA methods for all users ("users/$UserId/authentication/methods").

My environment has 10k users, querying the resource sequentially takes over an hour, so I made it multithreaded.
I use 16 in my 20 core system, that brings it down to ~30 mins. 
In another function in the same inventory script uses multithreading too, it lists all group members of ~7500 groups using "groups/$GroupId/members" that only takes 2 min and 15 seconds.

I played around with the number of cores, and barely get any 429's but still takes close to an hour.
I tried consistencylevel "Eventual" in the header, but had no effect. 

Is this difference caused by an issue in the MSgraph backend?
or is there something I can fix on my end to improve performance?

Cheers,

Sander