I recommend that you operate off the end of the array instead of the beginning. Run this to see the difference:

#define arraysize 1000

dimension a[arraysize]

for i = 1 to arraysize
	a = space(1024)
endfor

? 'beginning'
s = seconds()
for i = arraysize to 1 step -1
	adel(a, 1)
	dimension a[i]
endfor
? seconds() - s
	
	
dimension a[arraysize]

for i = 1 to arraysize
	a = space(1024)
endfor

? 'end'
s = seconds()
for i = arraysize to 1 step -1
	dimension a[i]
endfor
? seconds() - s