any way to boost data transmission in GPIB?
The original standard allows speeds up to 1 MBps, and the HS-488 standard allows up to 8 MBps. That being said, bus speed is sensitive to things like cable length, the number of devices on the bus, etc.

Most GPIB interface software (NI Measurement and Automation eXplorer, etc.) allow you to set the maximum speed and various latency adjustments for the GPIB interface. Try to shorten your cabling as short as possible and play with the interface settings to find the fastest stable speed your instrument can support.

If you have multiple instruments, it's best to wire them in a 'star' configuration with the host interface at the center (a separate cable from each instrument goes to the host interface) so that the minimum host-to-device distance is achieved. Sometimes geography and topography forces you to do other things like a daisy-chain bus with all the instruments in series. (Paralleling more than 3 GPIB cables can become unwieldly). 

It's also not a good idea to join GPIB cables in free space (that is to say, make your junctions at an instrument, don't just plug two cables into each other) since they tend to pull apart if they're not bolted to an instrument, and partially disconnected cables introduce all sorts of bus weirdness into the mix (corrupt data, hang-ups, etc.)

I've noticed that buses with many devices sometimes need their maximum speed set lower than those with fewer devices - most likely a signal integrity situation (total bus loading).