@Hepari00 mentioned about mouse too. So, it’s not a tablet problem. Also I’m afraid that lag may not appear that easy on powerful PCs with a lot of memory available. It’s really better to collect/monitor input events statistics along whole codebase i.e. max array sizes of all arrays along code path from APIs to very last receiver and also monitor memory usage while actively moving mouse. I’ll try to monitor memory usage myself and peek into the code.
Report: According to Task Manager in 1.3-beta3 and 1.2.27 Aseprite.exe memory usage grows about 1 Mb in several seconds of any active mouse movement (not drawing) over canvas and only over canvas and only with Pencil/Line/etc drawing tool active. Some memory definitely not freed!
Possible candidate: FreeHandController::movement calling Stroke::addPoint, while Stroke::reset almost never called @dacap