Since 2.6.3, the camera app take 10 seconds between starting camera app and be able to take a picture.
Tried with Open Camera, Fossify Camera and Libre Camera (all from F-droid):
Open Camera : same as preinstalled app.
Fossify Camera : instant as expected
Libre Camera : quite instant as expected (around half second lag)
device Info:
Google Pixel 3 XL aka crosshatch with 2.8 not rooted.
It is also on the FOSS version of OpenCamera (which has a pretty long ago last update). Not sure if there is a Play Store version (Edit: it behaves the same)
Anyhow, I think you need a different camera app if you wanna solve the problem fast.