Gyokko literally summons 10,000 flying killer fish to eat Muishiro from his pots, only for Mushiro to bisect them all in an instant.
Feat here: https://imgur.com/gallery/rt5Lk46
Muichiro Tokito is 160 cm or 1.6 m, and we can estimate his sword length and arm length from his character design:
Muichiro's Height: 1753 px = 1.6 m
Sword length: 908 px -> (908/1753) x 1.6 = 0.83 m
Arm length 564 px (564/1753) x 1.6 = 0.52 m
Sword + Arm length: 0.83 + 0.52 = 1.35 m
180 degree swing distance: pi x 1.35 = 4.24 m
Now to get the distance between Gyokko and Muichiro, and the height of Muichiro's jump:
Muichiro's Height: 231 px = 1.6 m
Distance to Gyokko: 1164 px -> (1164/231) x 1.6 = 8.06 m
Jump height: 209 px -> (209/231) x 1.6 = 1.45 m
The distance between Muichiro and the ground as he is landing can also be acquired from an other panel:
Muichiro's Height: 335 px = 1.6 m
Distance to ground: 76 px -> (76/335) x 1.6 = 0.36 m
So what is left is the timeframe. I could think of two methods and they are both lowballs considering that Gyokko was expecting the fish to be fast enough to successfully chase and overwhelm Muichiro.
I'll take the minimum height of Muichiro's jump, which is the height shown, and assume that Muichiro was slashing for the entire time until the panel where he is shown landing. We can get a timeframe from the distance divided by gravitational acceleration.
So the timeframe in this method would be: (1.45 - 0.36) / 9.81 = 0.11 s
Muichiro's attack range extends to several meters beyond the length of his sword, so it is possible that he occasionally cut more than one fish per swing. So I'll count 10,000 slashes, 5000 slashes, and a 1000 slash.
10,000: (4.24 x 10000) / 0.11 = 385454.55 m/s [Mach 1123.77]
5000: (4.24 x 5000) / 0.11 = 192727.27 m/s [Mach 561.89]
1000: (4.24 x 1000) / 0.11 = 38545.46 m/s [Mach 112.38]
Within this timeframe, for fish that are 2 meters away from Muichiro as he jumps would need to move about 18 m/s to reach him as he lands, and fish that are 3 meters away from Muichiro as he jumps would need to move at around 27 m/s. Which is a ridiculously low minimum speed since Gyokko is expecting the fish to be fast enough to successfully chase and overwhelm Muichiro.
I'll assume that the fish move at sonic speed, and the timeframe would be the time for the last batch of summoned fish to reach Muichiro's location from Gyokko's pots.
Spamming attacks stated to be at the speed of sound is done by low-mid tier characters such as Kyogai, but with sound speed it would be impossible for the fish to come close to chasing or overwhelming a Pillar-level opponent as Gyokko is expecting, as shown by these examples: Example 1 - Example 2 - Example 3 - Example 4 - Example 5. Gyokko was even made aware previously that Muichiro is faster than the average Pillar, so Gyokko wasn't under the impression that Muichiro was a fodder. Hence, sonic speed for the fish would be a reasonable low-ball since we know the fish are significantly faster than that by an unknown magnitude.
Hence, the timeframe would be: 8.06 / 343 = 0.0235 s
Counting 10,000, 5000, and 1000 slashes:
10,000: (4.24 x 10000) / 0.0235 = 1804255.32 m/s [Mach 5260.22]
5000: (4.24 x 5000) / 0.0235 = 902127.66 m/s [Mach 2630.11]
1000: (4.24 x 1000) / 0.0235 = 180425.53 m/s [Mach 526.02]
Other feats compatible with MHS+ speed: