In depth review of MLJModel loading time is necessary

[tlienart]

Importing MLJModels takes way longer than it should given that it doesn't really include any packages beyond the basic ones.

In view of the recent speedup achieved by tweaking how the metadata was retrieved, it would be useful to know what takes time when loading MLJModels and dig.

I suspect some of the work to reduce this latency will go in fixing things in MLJBase as well.

Help to do this would be very welcome (at this point it's just a case of figuring out what operation takes how much time when loading everything).

-- the first thing to check is that the actions where MLJModels retrieves the list of models and metadata is not super slow.

[tlienart]

Hehe, now it's ~4s 😄 most of which is MLJBase I think.

We can probably further improve this but for now I'm closing this.