ACLS is slow. It's only meant for complete resets of the display. To erase only the graphic page, use GCLS. Also, using LOOP ENDLOOP is preferable to using a GOTO label loop.
'acls at the beginning to reset the display ACLS LOOP 'gcls in the loop to erase the graphics. GCLS GBOX 100,100,200,200 VSYNC ENDLOOP