It's more footwork and consistency than speed though. Consistency throughout a fight is a quality fight fans don't seem to notice really but it's one of the most important for many greats, see Mayweather and Monzon too.
Marciano was so successful because of several factors: First he had the drive, will to win, heart, and chin of an ATG hwt. Very very few have this level of intrinsic attributes. Secondly fanatical conditioning. Third the ability to punch very hard. Fourth the ability to take punishment and not quit no matter what. This ties into point one. Finally he perfected his style. Marciano was a cagey swarmer in the ring with under appreciated abilities to block parry slip and counter. He was NOT the open target as many think of him. Sharpshooters like Walcott and Moore were very open that they would be able to hit Rocky at will but it never occurred.
His footwork was very good, whether that's down to speed or technique is immaterial but he was very rarely out of position or away from where he wanted to be. Consistency yes, you know against Marciano you're getting perpetual motion and 60 power shots a round, for 15 rounds. Why would anyone wanna face that?
Many. His defensive abilities are the most under appreciated of any past hwt champion. He is generally viewed as a slugger with a porous defense. This can be traced back to his time. Look at the comments of Walcott prior to their fight one. I believe Walcott calls Rocky an easy to hit "amateur and perhaps a "bum".
Twenty or more years ago I was a very vocal Marciano critic. But in depth review of him in action over the years changed my opinion. Marciano was the real deal. True ATG attributes that everyone should greatly admire.
"Street brawler" can be interpreted certainly. Marciano was no Conn or Tunney. He was an ATG fighter....a brawler with a nice level of finesse. Walcotts comments prior to fighting Rocky can be interpreted only one way. I always suggest reading Walcotts comments THEN with a critical eye watch the fight. Not saying Rocky was not being hit but watch the many times Walcott looks to load up and Rocky is not there to be hit. Marciano's back was like on hydraulics bobbing and weaving, moving in and then out of range. Making himself a small low target made him a tough fighter to nail cleanly. This is a forgotten art in an era where being as big as you can seems to impress the neophytes.
This is Louis's interpretation: "I didn't figure on any problems with Marciano,he was strong sure ,but he fought like a street brawler.Figured I could out box him anytime" "Yeah! I was right he was a a street brawler.He couldn't even touch me when it came to boxing". Marciano on Louis' jabs: " I couldn't avoid them I just had to take them. " If you want to be a stickler for accuracy,and you've mentioned Walcott's quotes several times now,you misquoted him. What Walcott actually said in reply to how he would do against Marciano was: "That guy? I guess he can punch but he can't box ,he's got two left feet, take my name out of the record books if I don't beat him". I first saw Marciano on film in the 1960's when I bought a collection of his fights from Blackhawk on super 8mm. I think by now I am familiar with his "style".
It's not black and white. He was great at slipping, bobbing, weaving, using parries and blocking. He was also great at being able to take a flush punches to the face, and keep coming forward. Video tomorrow!
Off the top of my head Moore Louis Walcott Liston Dempsey Robinson Wlad Lewis Tyson Foreman Foster Hearns