For me (Win 7 64-bit, SSD, 24 GB) the startup time increased more than three times from 17.3 s for Mma v9 to 58.2 s for Mma v10.
You can use
FE`Evaluate[FEPrivate`$StartTimes] // Column
wich I found here, to get more insight to the time consumption during startup.
In Mma 9 I get:
{
{"Initializing kernel connections..." -> 0.312},
{"Initializing cell styles..." -> 1.264},
{"Initializing fonts..." -> 1.389},
{"Initializing menus..." -> 1.404},
{"Initializing defaults notebook..." -> 1.545},
{"Initializing defaults clipboards..." -> 1.857},
{"Initializing internal preferences..." -> 1.935},
{"Initializing kernels..." -> 2.075},
{"Opening Welcome Screen..." -> 3.776},
{"Closing Splash Screen..." -> 4.821},
{"Preloading Favorites..." -> 4.836},
{"Handling CommandLineOptions..." -> 4.836},
{"Opening Files..." -> 4.836},
{"Do Startup Action..." -> 4.836},
{"Finished" -> 17.316}
}
and in Mma 10:
{
{"Initializing kernel connections..." -> 0.125},
{"Initializing cell styles..." -> 1.248},
{"Initializing fonts..." -> 1.388},
{"Initializing menus..." -> 1.404},
{"Initializing defaults notebook..." -> 1.56},
{"Initializing defaults clipboards..." -> 1.778},
{"Initializing internal preferences..." -> 1.825},
{"Initializing kernels..." -> 1.919},
{"Opening Welcome Screen..." -> 2.87},
{"Preloading Favorites..." -> 28.33},
{"Handling CommandLineOptions..." -> 28.33},
{"Opening Files..." -> 28.33},
{"Do Startup Action..." -> 58.204},
{"Closing Splash Screen..." -> 58.204},
{"Finished" -> 58.204}
}
Therefore the main problem seems to be
"Preloading Favorites", "Handling CommandLineOptions", "Opening Files", "Do Startup Action", and "Closing Splash Screen"
A hard reset by renaming the ...Roaming\Mathematica folder didn't help.
