I think they will transition all services that do not expose infrastructure (think i.e. Cognito, Pinpoint, VPC, Api Gateway, CloudFormation, ...) ASAP.
SP, like any standard platform (think i.e. SAP) invites the systems integrators to customize the crap out of it, which tends to impact the bad performance even more.
The JS API is total crap. One has to use it to develop anything that gets a bit more complex, but MS chose to ignore things that exists in the ES standard (like normal iterators, wtf) for quite some time to preserve backwards-compatibility with IE.
This make neither debugging nor any further exposure to the API any fun.