It's always speed imo. Even slow fighters get slower. Reflexes can be compensated for with guile, but once the speed goes, it's gone for good.
speed/reflexes. And for some reason the legs are the first part of the body to show the decline in speed and reflexes. Hearns used to move well when he was younger, and when he got older he did not move as much. By the time he fought Virgil Hill and Iran Barkley at lightheavyweight, he would not move much, he would just use his jab and control the action without too much leg movement-or as little as possible.
reflexes. You notice right away they just can't seem to fire off the shots they used to anymore. After that goes the legs when they just don't seem to be able to move the same anymore. After that depends on the fighter. Some people lose punch resistence while others seem to be able to take punches forever.