Hi,
We've changed the system like that because people were trying to add characteristics with hundreds of values and it was crashing the pages because of too much resources.
And generating all the variants and then deleting the uneeded ones, or not generating them, and adding the needed ones is not necessarily slower. It depends on your situation.
Finally, you don't need to add the variants one by one. After you've added your characteristics to the product, go in the variants tab, click on the add button, and there, for each characteristic, you can open the dropdown selector, press the CTRL key of your keyboard and click on the values you want to add for that product.
Then, click on the OK button and it will generate all the variants (combination of selected values of the characteristics) automatically.