There's a lot of players that have bad 40's but have good football speed and it's been proven. I don't know much about Glenn and how his football speed is but running a 4.64 40 doesn't mean he can't be a good CB in the NFL. Some CB's can make up for their straight line speed with quickness and good hips and feet to make for good cover abilities. It seems like we got some good names in UDFAency that had good college careers especially the DB's we signed but all of their biggest reasons they went undrafted was their 40 times.
I'm pretty sure that's exactly what it means.
I searched combine results for the past ten years and tried to find someone,
anyone, who has been able to stick on an NFL roster as a CB after being unable to run under 4.6 in the 40 yard dash.
I found literally one guy, Leonard Johnson, who made the Bucs roster as an UDFA in 2012 and even started some games in 2013 at CB due to injuries. Then again, I wouldn't exactly he's been a good player for them.
Also, last year, everyone talked about Stanley Jean-Baptiste being the next Richard Sherman even after he ran a 4.61 40 at the combine. The Rams (stupidly) drafted him in the second round. Now everyone's calling him a bust. Shocking.
In other interesting slow corner news, the 2015 Tampa Bay Fighting Lovie Smiths plan to start Johnthan Banks at CB, who ran a 4.59 40 (although he didn't count for my criteria since I limited my search to people who ran over 4.60).