The game engine is reaching its limits. A big reason why Tinkerer is focused on completing first the Greyport+Barony storyline (one more update expected) and then the Main storyline (Temple vs Tower, two updates expected for that).
What do you mean by "slow?" For me it shows a "now loading" image a lot more, before I might glimpse it going to another map and often not even that. But even now it never seems to be more than a couple-three seconds. Is it longer than that for you? Do you get lag for things like moving around or interacting with stuff?
It's not a shooter where lag is deadly so take that into account.

But if it's common even when not switching maps, that could be reallllllly annoying. You could try re-installing it from scratch and see if that makes a difference.