Rapp can play football, but quite honestly there are too many good safeties in this draft class who are just as physical who run a sub 4.5. The way the league is headed, if your strong safety can't play man coverage on slot receivers or tight ends, your team can be exploited in the pass with the wrong personnel on the field. This is why a player like Eric Murray was moved from slot corner to strong safety: A team is less likely to be exploited through the pass than the run with an aggressive hitter who can cover, even if he is undersized for the box.
Unfortunately for Rapp, his long speed was already a concern at the college level, causing him to play a deep "angel" type free safety. This could have been a scheme decision, but now that he ran like a high school level safety in the 40, we can no longer say it was scheme. Washington was hiding his liability.
That does matter.