I've seen this question asked and answered many, many times. And it seems that the answers depend on who you ask:

- WP Users: "I have no idea... it just is."

- Non-WP Users: "Because it's old/PHP. Use Next/Nuxt/Astro/Webflow/Wix/Ghost/...."

- Theme developers:
"It's plugins or hosting. Or other themes - ours is fast."

- Plugin developers:
"It's theme or hosting. Or other plugins - ours is fast."

- Hosting company: "It's plugins or theme. Or other hostings - come to us we're fast."

- WordPress "Experts": "WordPress powers 43% of the web(...)"

- WordPress Core Performance: "We'd like to make it fast, but we can't make any drastic changes, because someone should be able to run their blog on a solar-powered potato..."

- WordPress Performance Experts: "It's complicated... I can make an audit, it will take around 10h and cost $999"

- WordPress Cache Plugin developers: "It's not if you cache everything!"

- WordPress Performance Plugin developers: "There are many reasons, but our plugin fixes all of them!
*unless it doesn't - then you need to switch hosting"

( yes, it's a satire )