Hipersockets very much is a CPU function. In general, the faster the CPU, the faster the transfer.
Random thoughts ...
The processor and processor level you have. Both make a difference.
Is your microcode current for the processor you have?
Are any of the LPARs being used CPU limited?
Hipersockets is a CPU (microcode) function. Limiting CPU on an LPAR limits throughput.
Hipersockets is a synchronous method of transferring data. Sending data does not complete until the data is received. This is a big factor. If the receiving system is busy ...
QDIO is asynchronous. This does effect throughput. Testing I have done shows that QDIO OSA Express can be faster than using a Hipersocket connection.
Is z/VM in the picture?
If so, what z/VM and is it current?
There are APARs effecting Hipersocket performance under z/VM.
VSE. What version of VSE/ESA or z/VSE?
There are APARs related to performance for VSE/ESA 2.7 (DY46197 comes to mind).
This may or may not apply to z/VSE.
Are the hosts using Hipersocket devices on a separate subnet?
Are all Hipersocket hosts using the same large MTU
The maximum MTU size of a Hipersockets is determined in the IOCP Configuration of the IQD CHPID through the OS parameter.
OS = 00 : MTU = 8K
OS = 40 : MTU = 16KB
OS = 80 : MTU = 32KB
OS = C0 : MTU = 56KB
Choose the OS parameter carefully. The value you choose will be transferred for every transfer even if there is only 1K to transfer.
What is the PRTY of the FTP client/server partition?
Is it higher PRTY than the stack?
Are other batch jobs competing for CPU time?
What is the access method being used?
POWER is slowest, VSAM fastest.
What other jobs are running on the system? LPARs?
Other activity uses CPU that could be used by Hipersockets.