It's basically just number of possessions per game (or per 40 mins), which itself is a bit complicated to track and there's a couple of different formulas for it.
But crucially in this discussion, offensive rebounds are considered a continuation of the possession, not a new one. So even though we rebounded an incredible 42.3% of own misses last season, meaning that almost half of our possessions continued on past the initial shot attempt, we were still only #183 in average possession length on offense (KenPom's formula). If we were a really slow team on offense and rebounded that many of our own misses, you'd expect our offensive possession length to rank much lower.
As an example, here are the teams behind us in offensive rebounding percentage and their offensive possession length ranks:
#2 - Towson - 357th
#3 - St. Mary's - 359th
#4 - St. John's - 78th
#5 - SMU - 73rd
#6 - South Carolina St. - 220th
#7 - Purdue - 244th
#8 - Florida - 27th
#9 - Western Illinois - 342nd
#10 - High Point - 257th
So most of those do in fact rank very low, including three of the slowest offenses in the country. The ones that were still fast have a few things in common, but I'd say the most important is that they're average-to-good 2pt shooting teams and two of them were very tall. When they get the rebound, they're not trying to kick it back out, they're trying to score it. This is due in large part to just a couple of players. St. John's 6'11 center Joel Soriano averaged 6 offensive boards per game just by himself (incredible when compared to Garcia, who's known for offensive rebounding and had 4.3). Florida had a bit more rebound-by-committee approach because they have three huge guys, 6-11 Alex Condon, 6-10 Tyrese Samuel and the 7-1 Micah Handlogten who had an offensive rebounding percentage even higher than Garcia's.
Compare that with us, we were an absolutely terrible two point shooting team (#311). So we're missing more shots, getting even more offensive rebounds, continuing for even longer possessions, and yet we still hung in the 180's in offensive possession length.