Even though adventuron is technically 1.5mb, Html in modern browsers is now served gzipped, which means the engine compressed is about 370k. In addition, return visits are cached so no download, and JavaScript is only compiled the first visit them the compiled version is reloaded from the cache too. Should be quick even on slow mobile connections.
The editor gets slower with more assets, but you really need a lot to make it properly slow these days ( I sped it up by a factor of 20x recently). I can optimize it a lot more, but like I said in the jam notes, I'm aiming not to add a lot of features at this point in time. You really don't need a lot of assets in a text only jam.