Look in the Help > About JupyterLab menu entry. That shows the version of JupyterLab you are running. The current version is 2.1.2, IIRC.
I’ve seen browser freezing with very long notebooks with lots of cells, especially in older versions of JLab, but not with moderately sized text files (it seems you are mainly working with the editor?). Again, we’d love to see example files that freeze the UX to more clearly trace down the issues. Also it may be important what browser/version you are using.
I agree ideally a UX should be intuitive. VS Code uses a very similar sort of tab dragging to split panes that we do. Right-clicking on an editor or using the File menu to open a console connected to the editor automatically splits the pane. Would an editor toolbar button to implement the same thing (open a console in a split pane) have been more discoverable to you?
We did spend a lot of time to make the introductory docs extremely concise and helpful for common patterns, so you’d spend like 10-15 minutes rather than hours :).