So tired of this slow 4.2. I need the 4.0 os, which is afaik the one this phone came from the factory from.

I know, I know, you think I should do a reinstall from scratch. But this OS is just too painfully slow with a lot of contacts.

Those older legacy OS versions will probably only be available from the carriers' support sites. You'll be starting from scratch if you downgrade to 4.0, so trying a re-install of 4.2 might not be such a bad idea.