Nothing substantiated here, just something to consider.
FAQ and other sources show it's using OpenAI models but no specifics, the prediction speed seems consistent with GPT-4 and it has live web access now. I'm guessing there's a few more layers of logic being wrapped around Aria that take some time to gather and feed into the prompt going to OpenAI.
Fetching live grounding resources takes time, going out on a limb that Opera doesn't have the same $$$ to match Bing or Bard speed for that. ChatGPT free is fast but dumb and offline, Aria isn't bad for higher quality answers and live web access baked into a browser.
The way it embeds search links isn't displayed the same as GPT-4 generation so that filtering could also be part of the orchestration.
https://press.opera.com/2023/05/24/opera-unveils-integrated-browser-ai-aria/
'Aria, which is based on Opera’s own “Composer” infrastructure, connects to OpenAI’s GPT technology and is enhanced by additional capabilities such as adding live results from the web.'
Getting consistent subjective information is difficult, they(Aria, GPT-3.5, GPT-4) all seem to do a decent job when asking for the top x symphonies. GPT training data will likely have multiple opinions on the topic and it picks the best prediction from that given current context, abstraction and math
https://help.opera.com/en/browser-ai-faq/
'Conversation history is stored on Opera’s servers for 30 days. OpenAI’s servers will also keep anonymized (not connected to your identity) parts of conversations for 30 days. Open AI can also use questions you provide to the chat to improve their systems.'
I'm happy using Aria as an option, pop quick questions when I need internet enabled AI not tied to a large ecosystem or company. Also using Aria for cross checking another AI chat output or claim - let it crank out information in the sidebar then read when it's finished