A few comments (Disclaimer: I work at Unity):
-As others said, with Unity, you get a full game engine cross compiled to JavaScript/WASM, which has more overhead then simply rendering an image by other means (like img tag).
-We don't officially support mobiles for the Unity WebGL build target, because we believe that the platform (browsers and hardware) is not yet ready for this use case (running a full game engine compiled to the web) yet. That said, you might get better results with simpler engines which do support mobiles.
-WASM support on iOS is currently broken, so this probably would not work regardless of engine used: https://bugs.webkit.org/show_bug.cgi?id=181781#c4
(Try making a JavaScript release build instead)