WordPress multisite - Multiple databases or multiple database prefixes

I'm developing a test multisite website, mrrwebdev03.org and trying to import data from okobserver.net. I've been through countless WordPress.org articles on how to set up multisites. I have posts & media that span from year 2008 to 2016 which makes okobserver.net extremely slow.

Do I need to set WordPress up to use multiple databases (mrrwebdev03-2008, mrrwebdev03-2009, etc.) or do I need to use one database with multiple prefixes (wp_2008, wp_2009, etc.)? (I'm not sure how to do the multiple prefixes.)

I see several WPMU multisite plugins, but not sure if any of those would help in this instance.

Can you please assist? I know you all are the best in the business!
Thanks.
Bob