You can see in the save file size, that XCL save file sizes are under 1mb, mine are usually around 200kb. It used to be that Harlowe would continue storing every "turn" of data, but now it no longer does if you use the (forget-undos
etc macros.
XCL may not *seem* more complex from the outset, because it hides a lot of its complexity from the player and tries to present things simply. But if you take a look at the NPC generation etc under the hood, the cock database, the clothing database and sex position database, it is handing quite a large amount of stuff.
Most people do have good performance on Firefox, there are just so many things going into browser performance. My desired future state is to release an electron app that will have more access to system resources.