The easiest way to get it is for your boosting partner to quit as soon as the match starts (in "player's match"). He'll then search a custom game to rejoin the lobby (quicker than the host sending invite).
Now the loading time... It can go up to 1-2mn for getting into the lobby, then 1-2mn again for launching the match. But in fact it really depends on network connection between both players. Try to boost with a player within the same region: in the lobby, you'll see network bars like in your phone to see the quality of your network. 4 bars for both and the loading time should be minimal (10-15sec). 1 bar for a player doesn't bode well for the loading time...