I've watched more game footage and more All-22 footage of Free than any player ever. I've watched every snaps that he has played in the NFL at least 10 times. For many of his snaps it's way more than 10 times each.
Some season I had every nuance of every step that he took basically memorized.
I became obsessed trying to figure out why a guy that was 6-6, 325 with quick feet and good arm length, struggled so much. It just didn't make sense to me. Usually, an OL that struggles that much is either too slow footed or too smalll/short-armed but that was not the case for Free.
The short version is that he had a significant weakness in his upper body. He had no hand/arm strength once his arms were extended. It was very similar to a player with bad shoulders, but he never had surgery and it never got better. It all started in a game against the Niners when he was playing LT. The Niners DE Justin Smith would get his hands/arm under Free's and thrust upwards. Free had no ability to resist that move. Once it was on film, other teams exploited it. Then Free started trying to change his technique with things like keeping his arms extra low for as long as possible. This non-optimal technique obviously led to other problems. His lack of extended-arm strength, forced him to stay directly in front of defenders because he couldn't reach out to slow them with his arms away from his body. Again, it's very similar to someone with shoulder problems. Decent strength with elbows in, but once the elbows are away from the body, the arms become weak.
In later years people thought he was slow footed; however, in reality he was not. He just had to keep his body in front of defenders; whereas, most OTs could let defenders get outside of their body and still push them wide with their arms/hands.
Once OL coach Bill Calahan came to the Cowboys and veteran OG Brian Waters played some RG early that season, they devised a method to help Free. They played the RG with a shift towards the RT and away from the Center by a few inches. This allowed Free to over-set to the outside and not have to worry about getting beaten to the inside because of the over-set. This really helped Free improve from what he had been since Justin Smith had exposed his weakness. Callahan also adjusted the run blocking to put Free in situations that he could handle. There were a number of tricks they used tweak his responsibilities in the run game. One was to attempt to have Free block a LB as often as possible and minimize when he had to block a DE. Sometimes it was help from TEs. Other times they would have him help block Martin while somebody else blocked the DE. Sometimes Free would come back off that help block to pickup a LB. When he did run block a DE, they tried to make it wall-off type blocks. They would let the DE jump outside to contain the edge, which allow Free to just get in the way and the RB would run inside. Free didn't have to get any movement on the DE, he just had to wall-off much like pass blocking to keep the DE out of the play. Other times they actually used 2 TEs next to Free, had them combo block the DE while Free would help block with Martin and then kick out to get a LB when Martin had the DT under control.
It was incredible coaching if you really study it in extreme depth. The last year or so, Free did have foot problems which did slow him down a little and he really struggled because as I said before, he had to have good foot quickness to compensate for his other problems.
The reasons they liked him was that he executed the scheme without mental mistakes and he didn't miss games with injuries. He was good enough and Garrett places a high value on not making mental mistakes.
In 2014 when Parnell replaced Free for 7 games, they ran run plays to the right that they didn't run with Free. They loved to run the zone stretch left over and over and then come back with power right. Martin and Parnell would just obliterate people that were cheating over to defend the zone left plays when they came back and ran power right. When Free returned, those power right type plays would disappear.
Free was "good enough" to win many games, but he was never "good".