Change Font in Keynote is Super Slow.

Hello Bulawit.C,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with Keynote font changing slowly. Try the steps listed here to see if they resolve the issue: If an app freezes or quits unexpectedly on Mac - Apple Support

"When an app on your Mac stops working, try these suggestions.

• If an app doesn’t quit as expected, or it’s unresponsive, you can force quit the app (you may lose unsaved changes). Choose Apple menu  > Force Quit, select the app in the dialog that appears, then click Force Quit.
• If the app quit, reopen it by clicking Reopen in the dialog that appears.
• Restart your Mac by choosing Apple menu > Restart.
• Make sure the app is compatible with your version of macOS.
• Open another file in the app. There may be a problem with the original file.
• Check for updates to your software.
• Disconnect any peripheral devices you just connected (for example, a printer). They may not be compatible with your version of macOS.
• Uninstall any app plug-ins or enhancements from other manufacturers.
• If you recently updated the app or macOS, a plug-in or enhancement you installed may no longer be compatible.
• Diagnose problems with your Mac to learn whether hardware you installed, such as additional memory, is incompatible, improperly installed, or defective."

We hope this helps.

Cheers!