I requested this feat myself but I'm doing it now.
Distance[]
Homer's 1.75m (212px)
Distance to cannon is (262px) 2.16m.
Calculating the feat[]
I'll need the platform for the feat. Going by Homer's size (306px) it's 1.22m (214px).
Let's say the cannonball is similar to those used in the french navies which is 17.4cm.
Speed[]
I'm fairly sure that 200m/s is good to use. The cannonball was fired right in front of him after all.
Finding distance[]
The screen is 535px. The stage is at 70.6907383667003631 degrees or 0.86009m away from the camera.
For the cannonball (254px) I have 83.9039604913172514419 degrees or 0.096786m away from the camera.
0.86009-0.096786 = 0.763304m.
Note: Ignore the cyan line.
Finding the distance Homer moved[]
The stage is 1.22m (214px) so the distance Homer moved is 0.838m (148px).
I could end it here, but there's more! In the very next frame Homer moved more while the cannonball is frozen. Funnily enough, it's 147px which is 0.838m as well (the difference is meaningless since the number is followed by a zero).
So let's add the results, finding 1.676m as the final distance.
Finding speed[]
Okay so this is pretty simple:
(200m/s*1.676m)/0.763304m = 439.1435129384884659m/s or Mach 1.28 Supersonic
Nice feat.
Ugarik's version[]
Stage screen distance: 1.22*535/[214*2tan(70/2 deg)] = 2.18 m
Cannonball screen distance: 0.174*535/[254*2tan(70/2 deg)] = 0.26 m
Cannonball-stage distance: 2.18 - 0.26 = 1.92 m
Speed: (200*1.676)/1.92 = 174.58 m/s (Subsonic+)