In general, you can probably assume that if the next byte you want isn't in the same sector (either 512B or 4kiB) as the one you just read, the drive is going to have to rotate again to read it and possibly move the read head to another track. That takes somewhere on the order of 10 milliseconds - a little less for fast drives, a little more for slow drives. You get slightly better performance if the data is all fairly local to each other, as the shorter travel for the drive head can help, but it's not a huge effect. If you need to wait for the drive to spin, it will only spin so fast, after all.
If you can section your reads up into somewhat bigger sequential chunks, you'll be far better off - the difference in throughput between 4kB random reads and 128kB random reads on spinning disk is in the neighborhood of a hundred-fold. The first few entries on this graph
should give you some idea, bearing in mind that the VelociRaptors are high end 10k RPM drives, as opposed to the 7200RPM of most desktop HDDs or 5400-ish RPM of most laptop drives.
FWIW, fragmentation is becoming a non issue on modem systems, as I think all current OSs take care of it in the background by default. So you can assume what is a sequential read to the file system will almost certainly still be sequential when it hits the disk.