Hi mate,
The speed for DeepL is completely out of my control. What you can do is to pay for DeepL Pro, supposedly Pro users have their translation requests prioritized. I have not tried it myself so I don't actually know if it makes a big difference during both peak-time usage and non-peak-time usage.
I am not 100% that the "Translation failed" message is due to overloaded DeepL servers, but it does sound likely.