Synchronize for mobiles freezes Lightroom ?

Hi !

I use last Lightroom classic CC.

With a MacBook Pro with MacOS last version too: High Sierra : 10.13

I have 16 GB of RAM

An external disk is a thunderbolt one.

Why Does it take so much time to Synchronize My 19.000 photos for iPad Pro ? This Synchronize operation seems to freeze LR ? RAM Full ! Time machine 100% ?!?

Thank you for any advice or support.

Is it possible to "Synchronize" first the whole Library in a folder and copy it on my iPad Pro ? And then after create a synchronize task ?

Best,

Pascal