I'll explain this via scaling chains with scans
And while gathering scans I noticed do agree that strength where it relates to South does seemingly equate to both Strength in speed but as you'll see that'll change things up a bit
A casual South gets blitzed by Waka and Benkei both of whom Akashi considers to be relative to Senju herself as he believes she underestimates their strength (all of this is shown in the scans)
Anyways speedwise this casual South couldn't keep up with them but when he's fueled by dark impulses(similarly to Mikey) and gives into his violent urges it makes him stronger(again going by the notion strength is determined as a whole of stats including speed namely) which in turn allows him to fodderize Waka and Benkei without them even being able to react
After this we of course get South vs Senju with the first half of this fight putting this South against Senju who after getting tagged by South has to realize her potential to stand a chance
However once again South gets fired up "for the first time in a while" (this being verbatim from his own words even calling Senju by her title which shows she performs better than Waka and Benkei) unfortunately however Senju doesn't really stand a chance against this fired up South and only gets to dodge one blow, throwing a kick before being immediately tagged while in motion swatting her away which puts her out the fight
This fired up South is approached by Mikey who in that same chapter LETS HIMSELF GET HIT by Kakucho not even being concerned with the blow
https://imgur.com/a/QrcNd0Z
Mikey isn't fired up and feeling the rhythm of battle the same way South is and instead is fighting while giving in to his dark impulses which Senju even notes
https://imgur.com/a/ab9wMvp
Anyways this fired up South and Mikey trade one blow before they then exchange a barrage of blows with Mikey absolutely destroying South without him even being able to realize the amount of damage he's taken in the brief but quick exchange with Mikey fully dominating
https://imgur.com/a/a2CD4PU
https://imgur.com/a/964Tsqe
This scaling chain fits narratively from the battles of the 3 deities and speedwise from the way this went but here it is
Casual South<Waka and Benkei~Senju< South giving into his dark impulses(upon realizing he could be beaten in his battle) <Senju after realizing her potential<Fired up South<<<Dark Impulse Mikey<<Serious Mikey
With this I'd say it's pretty clear Mikey is above them in speed and just on this scale in general
Again though this same Mikey was willing to take hits from Kakucho without caring much and pretty blatantly didn't really care with South either as he dominated him completely and was slowly beating him to death as he lay on the ground post their fight
Mikey in his fight with Izana fought much more seriously and harder with Izana with everything he had basically while he's clearly dominating here without even a question to his victory, south's strength or the fact that he's going to kill South. Mikey's portrayed as being pretty disconnected and casual here post draken's death which some characters even question as they figure he'd be more fired up and pissed about it