Will having two administrator accounts on a Mac cause it to start slow?

No. That should have no effect at all on day-to-day performance.

In fact, it's not a bad idea to have a second admin account to fall back on should something happen to the first.

Download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

Reply Helpful

Please post the report in your reply as described in my last post.