In the last two days, I had a look to this bug:https://bugs.launchpad.net/ubuntu-clock-app/+bug/1199838
I run some tests, with/without a workerscript, and with/without the animation on the analogFace.
I found that there's the same problem: the animations make timer ticking slow down.
I have a 2012 Nexus 7, which had several problems with the older version of Mir.
The workaround was to use SurfaceFlinger on this device.
So I thought to run another test: same clock-app version, but using an old Saucy build with SurfaceFlinger.
I also had a look at the CPU usage on the device, using htop from an adb shell.
I saw two things:
1) stopwatch tenth of seconds were at regular intervals (they weren't on Mir)
2) the CPU usage seemed to be higher on Mir.
I think the problems in the clock-app are not related to the app itself, but related to Mir.