Hmmm… I would expect each 10K import to take the same amount of time, not increase. I can not think of any reason off the top of my head which would cause this.
Go ahead and contact me via the Contact link at the very bottom of the page so we can take the discussion to email and get this resolved much quicker for you. I’m going to want a copy of the 100K and the 10K chucks CSV files which you be able to send me when I respond to your contact form submission. I’ll also want a screenshot showing the mapping that you are using that way I can duplicate your import and hopefully track down the code logic that is causing performance issues.
Also, just so I am on the same page, your server, it is dedicated, correct? So nothing else can be taking resources when you are importing?
As for a direct import into the database, the columns for address, phone and such are serialized arrays of the data stored in the separate tables. These are used as caches so during display of the directory only the main tale needs hit with a query vs making many, many queries to multiple tables.
Looking forward to hearing back and resolving this.