CiviCRM Slow Bulk Emails to Sparkpost via Rest
How many emails are you trying to send? mailer cron=8 is really if you want to send a lot (I would not consider putting something else than 1 if less than 100k emails sent).

As for the job size at 1000. you are on the lower side, and would probably put at least an order of magnitude higher.

With more "normal" settings, you should get a better behaviour.

This being said:
1) yes, sending a mailing means a lot of traffic around your db, it's normal, nothing to worry about, it's big queries to record which emails have been sent

2) yes, each email is sent a separate email, here again, it's normal and works as intended

What is less normal is the number of connections. If you use smtp, it will create one connection and push all the emails of the batch in this connection. Obvioulsy the more cron jobs running you have, the more connection, hence my recommendation to lower it to 1 to see how it goes

I'm not familiar with the sparkpost extension, it might use a http rest api, and depending how it's designed, make more requests, you should check the code or look directly with the authors of the extension.