| it takes about 4 seconds between exposures.
I never knew Minolta can be this slow, my brother's 3mp Minolta
is fstr. I thought only cheap brands like Genius can be this slow.

| I presume this lag is due to the time taken to load
| the image into the SD card.
No, the built-in microprocessor takes too long to compress image
into a jpeg file. Try scan & defrag your card 1st.

| If I buy an "Ultra" SD
| card (such as sold by Sandisk) will this achieve that?
unlikely

| is it really the card or is it a natural limitation of the camera
take camera to a pc fair / shop where SD cards are on demo, try
a "fstr card" & see