Hello, I've been on and off lurking since the days of nesvideos, so uhm hi!
I've noticed something that might save time in the 100% soul run: L and R let you page-up or page-down in the soul and weapon menus. This would make switching souls (particularly during the Graham fight) a handful of frames faster. Also switching equipment may be faster too, I haven't looked to see how much filthy loot Soma ends up with in Kriole's run :)
See here for an example of changing to the magic set on a character with all of the souls. Here is a frame-by-frame analysis:
41 S - and we're off!
52 A - enter soul menu
69 A - enter bullet souls
70 R - page down
73 R - page
76 > - right
77 v - down
79 R - page down
82 R - page down
85 R - page down
88 v - down
90 v - down
92 A - select flame demon
95 v - select guardian souls
98 A - enter guardian souls
99 R - page down
102 ^ - up
105 A - select giant bat
108 v - select enchant souls
111 A - enter enchant souls
112 R - page down
115 > - right
118 A - select succubus
121 B - out of soul menu
137 B - out of pause menu
As you can see, when you scroll the menu, you generally have 2 empty frames before input will be recognised, while if you don't, you can get new input straight away. As a result, while flame demon and succubus are faster using R to page-down, giant bat is actually the same speed: [R..^..] takes 6 frames because of menu scrolling, while [v.v.v.] is three button presses but with no scrolling still takes 6 frames. I'm not sure why sometimes you can input on the immediate next frame (frame 77 above) and why sometimes you need to delay one frame (frame 79 or frame 92). I have also seen [R..R] (ie R, no input, no input, R) not recognised but [R..v] recognised in the same situation. There's some lag that I don't understand caused by this menu.
I wonder if one or two frames can still be shaved off by reordering the menu commands. I hope this is helpful!
EDIT: Uploaded the wrong video the first time.