Like
@AK47 said, NetSuite won't be that quick no matter what and a simple customer record would for sure be the fastest. You can either use NetSuite's csv uploader, but it's limited to 25k rows per file so you would have to chunk up the file each week. You could also use Celigo and there are a couple options. We have a data loader (
https://docs.celigo.com/hc/en-us/articles/226949488-Create-a-Data-Loader-flow) which is free and it would let you just select the whole csv and not have to chunk it up. This still requires you to manually upload it each week. The other option would to automate it in someway by using normal Celigo flows and to pull the file from S3, ftp, mysql, or whatever. This would require a celigo license, but you then wouldn't have to touch it each week.
As for performance, we would process the file up to specified concurrency limit you set on the connection in Celigo. For example, if you only let us use a concurrency of 5, that's how fast we would go. If you are on a low NetSuite tier or don't have SuiteCloud+ licenses added, you may only have 10-15 concurrency max anyways which would the limit for any solution.