Thanks goes to Nitsuja and Upthorn for their Sonic runs, Orkal for his glitch videos, to Mr_Sweed for the Launch Base 2 improvement and to you for watching.
In Marble Garden 2, there is
one of Robotnik's attack patterns which allows a 1 frame improvement; it takes much longer to manipulate it (with pauses or by loitering) than it is worth, unless one were to gain enough frames in the preceding levels to manipulate this attack pattern (
not an easy task).
The pauses at the start of Sky Sanctuary may be removed if enough frames are gained in the preceding levels. As is, all of the boss attack patterns cost about 20 frames with perfect play in in-game time. Those 5 frames would be a real-time improvement only.
Likewise, if one were to replay the Death Egg 1 boss, up to 16 frames in real time (but none in in-game time) might be saved by removing the pauses at the end of Sky Sanctuary. However, most of the boss attack patterns in that range are highly unfavorable and probably slower to kill; in all likelihood, at most about 6 frames of pauses might be gained by replaying the fight. Again, this would only be a gain in real time; in in-game time, no frames can be gained from this.
With the same input and the same position (right down to the same subpixel position), the death at the start of Hydrocity 2 can be 1 frame faster -- Sonic even dies at the same frame, but the timer stops 1 frame earlier. Likewise, the post-Launch Base 2 cutscene can be 15 frames faster by standing in the same exact position. I don't know what causes these differences, but it is probably related to cumulative differences in execution, as they usually occur when the preceding levels have changed in length (up or down). If one were to gain "enough" frames in the preceding levels (again,
not an easy task), these "extra" frames
might simply just go away; I have no idea what would constitute "enough" frames for this to happen, though.
I am not aware of any other improvements at this time.
marzojr: Canceling to look for improvements