NFL Network host Rich Eisen -- who clocked the worst 40-yard dash time at the 2014 NFL Combine -- is finally talking about his record setting performance, telling TMZ Sports ... "I only know one speed ... and it's slow."

Eisen's sprint has become a tradition at the combine dating back to 2005 -- when Terrell Davis challenged the anchor to get his ass out from behind the desk and hit the field.

This year, Rich finally broke the 6-second mark ... with an official 5.98, shattering his personal best by half-a-second ... and when we saw him at LAX, he couldn't have been more proud!!

Just for perspective ... the next slowest 40 time this year was 5.48 ... recorded by a guy who's 6'6" and weighs 311 lbs.