Kind of a chicken and egg problem as a webgl build lets people play and not worry about downloads. My suggestion get the latest build of Unity for your game and dig out those webgl build bugs and I think you will get way more feedback.
If things run slow on WebGL you can use the profiling tools in Unity to highlight what is slowing your game down and you can optimise those areas, for instance just reducing the amount of things in the game, changing missiles/bullets from game objects to particles. Or sometime enemy AI does not need to recalculate everything every frame as moving towards the player is enough and additional steering or attack logic could be every other frame.
Or if your units use individual classes you could turn them into super lite classes that just register with a Hord manager class that can more quickly process their actions with one Update function looping over all of the enemies as opposed to 10 or 100 Update function calls every frame.