Ensuring good performance is not yet covered by the docs, but let me just tell you that the FILL mode is a potential CPU hog!! (No wonder really, as it’s writing dozens or even hundreds of lines several times per second…). Did you enable this when it happened?
In general, retriggering notes is a bit heavy and could probably be optimized some more - especially when the retrig count is high.
Edit: while on the subject of things not covered by the documentation, I did a bit of a hackish workaround for the virtual keyboard: if you’re using a keyboard layout other than english, you might experience wrong notes when inputting multiple notes while holding shift (transpose is working OK). This is due to a limitation in the API described here.