Speeding up readpixelfast?

you can guess it when you measure the time of both, that's pretty simple. something like:

t=createtexture(512,512)
setbuffer texturebuffer(t)
t1=millisecs()
lockbuffer()
for i=0 to 100000
rgb=readpixelfast(rand(0,511),rand(0,511))
writepixelfast rand(0,511),rand(0,511),rand(0,$FFFFFF)
next
unlockbuffer()
t2=millisecs()
time1=t2-t1
freetexture t

t=createtexture(512,512,256)
setbuffer texturebuffer(t)
t1=millisecs()
lockbuffer()
for i=0 to 100000
rgb=readpixelfast(rand(0,511),rand(0,511))
writepixelfast rand(0,511),rand(0,511),rand(0,$FFFFFF)
next
unlockbuffer()
t2=millisecs()
time2=t2-t1
freetexture t

if time2<time1 then 
 print "flag 256 is faster"
else
 print "flag 256 is slower"
endif

The difrence between them is about 70% on my machine. So an image takes 130 ms, a texture with the 256 flag as well, without the flag only about 40ms.

I am still confused since I think I clearly remember the flag 256 once used to speed pixel access up massively on exactly THIS machine.