Well bummer that. I always assumed (since I got it back in 89) that my US Doubler was exactly 3X faster. I've been lied to for the past 3 decades.
It may still be. This is purely the rate of the serial transfer though. Aside from that, the US Doubler's optimized sector skew about was about 13% faster even for standard skew disks when used at 1x SIO, reducing the inter-sector wait time even for those then using the disks in a stock 1050. (This same skew was seen earlier in the 810 archiver, also created by Mike Gustafson - the precurser to the US Doubler ) There may also be optimizations with inter-track first sector start/skew as well, so that it can read the first sector of the next track with minimal rotational delay. With that in mind, the real-world throughput of a US Doubler with an UltraSpeed skew disk may indeed be 3x faster in practice.
Time for me to break out RWTEST tonight to see
I did confirm in the past that for pure sequential reads/writes, the USDoubler with ultraspeed skew beats the happy marginally because the happy takes extra time to buffer whole tracks, and buffered track writes also incur an additional rotation to verify.
Edited by Nezgar, Mon Jun 11, 2018 3:31 PM.