Hi. First you need to buy all DLC require from what cars you subscribed. After you bought DLC, steam ask you if you want to install them. Install and then your game shoud work. If you subscribe to all cars, the game will load many minutes before to enter in game. You can have ultimate PC configuration and the load will be the same; this is a problem from game and creators needs to fix it. I suggest to subscribe only for your favorite car if you don't want to wait any longer to enter in game.
And when you play, DON'T gather tons of items in inventory. The game will go worse. You will have much time LAG, until you free your inventory. Keep only few things and the rest of items buy it from shop in that moment. Configure controls to have easy acces to shop.
I choose Q button to enter in shop quickly when parts are in grey. And TAB button for inventory. It's very easy in this way.