Not got an SSD atm, looking on grabbing one soon. Out of all the games I have, DOOM seems to only give me this problem, so it cant be the HDD. Suppose an SSD would be the fix to speed up the load times, but surely a standard HDD would be fast enough to boot a game to the menu.
Just set +com_gameMode 1 +com_restarted 1 in launch options so that it boots straight to single player, kind of loaded quicker than before hand.
Remember that doom is massive, around 70GB. In general bigger games take longer to load, much bigger games take much longer.
Might be worth checking that the game files aren't fragmented across your HDD.