checkpoint duration IDS 9.40

Hi, we use IDS9.40FC2 and we have 8Go informix BUFFERS (4096000 in ONCONFIG). We are working on a 6 processors computer with 12 Go memory and we have fast disks (22 Mo/s writing during checkpoint). Even if there is few data to flush while checkpoint occurs, checkpoint duration is at least 3 seconds. I tested with just one dirty page and its duration is alway 3 seconds. This duration grows up or decreases proportionnaly with BUFFERS. Does someone have a idea ? Is it normal ? About what i read, checkpoint duration is dirty pages dependent but not BUFFER size dependent. Am i right ? Thank you.