The git project contains quick checks (scans) on ways to improve system stability when it comes to low latency and real-time applications. This can improve jack to where the buffer size can be as low as 128x2 for USB class 2 audio hardware using a pre-compiled low-latency kernel. Should be lower (64x2?) for a custom compiled real-time kernel.. Otherwise using standard kernel you might still have issues at 1024x2 with a busy system.
See:
git clone https://github.com/raboof/realtimeconfigquickscan.git