Hi there @flipgatt,
No, 10 to 20 seconds to load the subscribers list would not be expected behavior with MailPoet. The page load time would depend partly on your server environment/resources, and partly on your site itself (plugins, theme, etc).
Checking on my personal test site with 600 subscribers/emails, I can refresh the page, and it’ll load the page + subscriber list within a couple of seconds:

Link to image: https://d.pr/i/o0rgKd
As a first step, I’d recommend a conflict test to see if there’s any other plugin/theme code running that is slowing things down. Testing to see if the issue occurs with only MailPoet active, and a default theme like Twenty Twenty would help to rule out any conflicts. We have a guide for conflict testing available here:
https://kb.mailpoet.com/article/204-how-to-test-for-plugins-conflict
You could also install Query Monitor to help narrow down where specifically the slowdown is coming from: https://www.ads-software.com/plugins/query-monitor/
Please let me know how that goes, or if you have any questions!