Finding a working link for the Opera Mini .xap file for Windows 10 Mobile is difficult because Opera officially discontinued and removed the app from the Microsoft Store.
Check XDA Developers forum threads titled "[APP] Opera Mini for WP8.1 – sideload on W10M". The links there are verified by mods. Do not trust random blog comments. Use the search term OperaMini_15.0.2254.53930_ARM.xip (Note: some archives incorrectly label .xip, rename to .xap). opera mini xap for windows 10 mobile link
Since development has stopped, you must rely on third-party archives to find the necessary installation files: Finding a working link for the Opera Mini
By stripping away heavy scripts, pages load faster on older processors. Do not trust random blog comments
Q: Is Opera Mini XAP compatible with Windows 10 Mobile? A: Yes, Opera Mini XAP is compatible with Windows 10 Mobile.