Are the LrCat files really 400 GB in size, or is that including the preview folders?
If the LrCat files are really that big, I would leave them as separate catalogs for performance reasons unless you really have a need to merge them.
Slow importing can be a hardware problem, perhaps a bad card reader/bad USB cable/bad connection to the camera or to the USB port. I would certainly investigate this, and try importing with different hardware.
Try aborting the import, closing LR, and rebooting the computer. See if that speeds things up.
I eventually managed to consolidate all my catalogues in a master, now 3.4GB in total size and roughly 190K images.
Performance seem OK.
I decided to break down the catalog that required several days to complete the import process in four smaller portions of almost identical dimensions. This solution dramatically shortened the entire process. What will remain a mystery is why two of the four new catalog required several hours each, while the other two completed the process in a matter of minutes.
Had a conversation with Richard Curtis from Adobe at the Birmingham Photography Show and he strongly recommended to create a single catalog with Smart Previews (he claims to have a Master catalog with 300+K images).
Waiting for Apple to introduce new high performance iMacs with Type C ports, to further improve performance.
Our catalog is 943315 images but it is getting dang slow on imports from other catalogs because it tries to compare each image agains the ones that are in db.