OK thanks, I will try with the GIT version! Transfer time with a 4MB image is significantly less than 0.5s when writing directly to an SSD, even using CFITSIO . Well anyway for now the limiting factor seems to be the ASIGetExpStatus() issue.
btw I hope you will be able to reproduce the "Failed to save file to" bug with large sequences in the "Upload: Client" mode. In practice this is the most annoying issue with high acquisition rates.