In Safari, there is a lag for the Open Location and New Tab shortcuts.

Launch "Console" and have it running side by side with Safari. See if entries about "IMKServer" appear when the delay happens:

If you see similar entries try following below: https://gist.github.com/wkrsz/3d5ff7f908a180b87498

Steps from the article:

1. Reboot into system restore mode (+R immediately after boot)
2. Navigate to Utilities > Terminal
3. Enter the command csrutil disable
4. Reboot
5. Open Terminal
6. defaults write -g ApplePressAndHoldEnabled -bool false && launchctl remove $(launchctl list | grep -oE 'com.apple.PressAndHold.+$')
7. sudo chmod 000 /System/Library/Input\ Methods/PressAndHold.app/Contents/MacOS/PressAndHold
8. Reboot
9. You can choose reboot back into restore mode and issue crsutil enable again to re-enable rootless mode.

It's originally from Github user squeeb: https://gist.github.com/squeeb/5a9dd082cae1b2d753a0 – I just forked the article to fix a typo in one command.

There are a few things you can try:

• Turn off iCloud sync for Safari
• Set "New tabs open with:" to "Empty Page"
• But for me, the culprit was "Retab" Safari extension.