The mid-level processor on the Pixel 4a may just not be performing to your expectations. A phone with a high-end processor would perform better. For GrapheneOS, the fastest compatible phone available (used/refurbished) right now is the Pixel 4 (or Pixel 4 XL).
Also, if you are using a Pixel phone with a non-default flavor of Android, the Google Camera app still works if you download it manually. APKMirror is a trustworthy app source run by Android Police:
Pixel 4 running graphene. I'm sure it's fine by android standards, but if you're used to iOS, it is unbearable.
Going back to iPhone as soon as I've got some free time to get everything set up again.
Unrelated, but I'm still very surprised there's no standard way of doing live photos on Android. They really do add a lot to the experience of reviewing old memories and Google has had at least 5 years to catch up.
On my Nexus 6P I use GCam v. 5.2.019.188906351 and it performs really great! It is quite slow with HDR+ (but usable), and almost on par with the default camera without HDR+ (still producing great camera quality). I am curious whether the experience is similar on a Pixel line, with Lineage OS (or any other custom ROM).
Also, if you are using a Pixel phone with a non-default flavor of Android, the Google Camera app still works if you download it manually. APKMirror is a trustworthy app source run by Android Police:
https://www.apkmirror.com/apk/google-inc/camera/
(For Pixel phones using an older Android version, you may have to use an older version of Google Camera if the current version does not work.)