Hmmm, if you're still getting slowdowns when using joined sprites, something else is causing the slowdowns. Sprites are more or less free to display, so the slowdown must be caused by the game logic somewhere.
Are you doing any multiplication or division? They're very slow on a 68000 and much faster on a 68020. Also, make sure you're defining variables as words or bytes where possible, because Blitz uses "quick" variables by default and these are also much slower to deal with on a 68000 than a 68020.
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