There are many factors why you're WordPress website is slow. Here are some tips and reminders to keep in mind to make your website fast!

1. Assess the speed of your website and address the issues. Use Google PageSpeed Insights 
2. Choose a web hosting plan that suits your website needs. There are times that to make your website fast is that you might need to upgrade your web hosting from Shared Hosting to VPS or to a Dedicated Server, because your Web Host is like a Store Place, where when you started, that Store Place can only accomodate up to 100 people. But as your business grows, that same place cannot fit 500 people or maybe it can but it makes everyone have a hard time to move. It is the also the same with Hosting your WordPress website. It must be adjusted to the resources it needs to accomodate and serve better your website visitors.
3. Make sure your images, and codes are optimized for speed. You may use EWWW Image Optimizer to optimize your images. For Optimizing your website codes, you might need to hire a website developer.
4. Use a Caching Plugin like WP Super Cache or other caching plugins out there. 
5. Use a Content Delivery Network (CDN). You may use CloudFlare CDN.

Here are some good reads on how to make your WordPress website fast:

• https://makeawebsitehub.com/speed-up-wordpress/
• https://premium.wpmudev.org/blog/speeding-up-wordpress/