There is a limit on iOS, yes. It is due to how much can fit in the graphics chip memory, and varies from one device to the next. It is quite big.
The lag you are seeing is unrelated though. That is just the processing of the changes you are making, which can slow things down for big notes. It is something we are definitely planning to improve when we get the chance. For now, a good strategy is just to break the notes up a bit into smaller ones. This also makes things generally more organizable and navigable.