For the hard drive slot, just get the IDE to CF adaptor, costs $9 or less, then get the fastest CF card you can.
SSD will work but LampDog is right, (What’s up Lamp!) there is a limit internally, even for CF.
If you don’t have one already, get a USB card reader for your computer and skip using the MPC’s USB connector, much faster.
I got the fastest microSD card I could (160mb/s maximum) and a mSD to CF adaptor, make sure you get fast read and write speeds, you can use up to a 128GB card with JJ OS. I use a 16 gb in the card slot that has 60mb/s speed, but neither load everything in seconds.
Loading takes a minute or two maximum for my projects with memory almost maxed out, never really was a problem but with this you get the fastest speeds.
Maybe an SSD is faster, if you have one on hand try it, but I would go with the internal adaptor and fastest CF/microSD cards you can.
Using packed programs speeds up saving and loading.
Upgrading the motherboard for faster speeds is probably impossible or would cause other problems.
What are you loading/saving that you want so much speed?
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