[TYPO3-windows] im 4.29 slows down

[daniel]

hy list,
we use the following  system:
win 2003 server
apache 2.2.0
php 5.1.2
mysql 5.0.20

some records of type "image" consist of 8 pics which are calculated down 
to 600px allover width
if we use im4.29 system slows down dramaticly (takes about 2minutes to 
render), im 6.28 system runs fine but generates very big images (e.g. 
pic with 145px x 75px) 30kb!!!

does anybody know why im4.29 slows down ?
or why im 6.28 generates so big imagesizes ?

thanks alot for any hint

daniel