Updates usually contain many changes and while the main file is downloaded quite fast, other files like graphics for slots are downloaded at the moment when a player wants to play a given game.
If you try to start a few slots at a time, files are queued for download and if the first game loads, there are still files downloading in the background. And that uses the bandwidth and may cause issues during playing.
Solution? After the update, wait patiently for the game of your choice to fully download. If the loading last more than 2-3 minutes, restart the app and try again.