If I’m not mistaken, it was introduced in one of the last releases. If needed you could deactivate the compressed feature as follow
echo 'Acquire::GzipIndexes "false";' > /etc/apt/apt.conf.d/98dietpi-uncompressed
/boot/dietpi/func/dietpi-set_software apt-cache clean
apt update
On my RPI3B+, apt update execution time reduced from 20s down to 6s
Before
root@DietPi3:~# time apt update
Hit:1 http://raspbian.raspberrypi.org/raspbian buster InRelease
Hit:2 https://archive.raspberrypi.org/debian buster InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see it.
real 0m19.475s
user 0m19.106s
sys 0m0.449s
root@DietPi3:~#
After
root@DietPi3:/etc/apt/apt.conf.d# time apt update
Hit:1 https://archive.raspberrypi.org/debian buster InRelease
Hit:2 http://raspbian.raspberrypi.org/raspbian buster InRelease
Reading package lists... Done
Building dependency tree
Reading state information... Done
1 package can be upgraded. Run 'apt list --upgradable' to see it.
real 0m6.336s
user 0m5.005s
sys 0m0.496s
root@DietPi3:/etc/apt/apt.conf.d#