By itself, Flash content won't slow down your website as long as it's been coded properly and has a reasonable download size. I would recommend trying to understand what is actually slowing your website down. There are a number of ways to do this and guides to help you understand website loading speed and techniques. Here are a few:
https://developers.google.com/speed/pagespeed/insights/
YSlow - Official Open Source Project Website
Website speed test
And a guide that gives background information and strategies - A Beginner’s Guide to Website Speed Optimization by Kinsta
That said, you should consider your customers, the platforms that they use, and what technologies are available to present the content you require. Do people visit your site with mobile devices? Is there functionality that Flash provides that is not available with other HTML5 APIs? As with most decisions, make sure you pick the right tool for the job.
Hope this helps,
Chris