Catalogue very slow to transfer to Kindle

[Jacques Q.]

Hello.
I love reading on a Kindle more than any other device and have all my documents (about 60000 of them, in around 4000 collections) stored in Amazon's cloud, but Amazon has always had that weird (and IMHO completely outdated) policy of not allowing more than 15000 titles (ebooks, documents, collections...) to be listed on the device - meaning that the older documents disappear from sight when they're over 15000 uploads "old".
So I regularly update a catalogue, both by authors and titles, in Calibre, and send it to my devices, so I can easily find books not appearing on the devices directly, and go send them from my PC through SendToKindle if I want to read one that is not presently listed on the device.
But I've noticed that trasferring the Catalogue to Amazon's cloud + devices takes a very long time : while it's "only" a 2.6 Mb file, it will usually take more than 20 minutes to transfer, while I frequently send much larger EPubs that take way, way less time to reach their destination.
Does anyone have an idea why this is, and possibly how to remedy that slowness ?