Description of problem:
dnf-2 is much slower than dnf-1 when handling groups And a few other things. Way much slower.

Version-Release number of selected component (if applicable):
=============================================================
dnf-2.7.3-1.fc27.noarch

How reproducible:
=================
I'm experiencing it with quite simple installations with mate-desktop.
Some of them are brand new fc27 machines, some others are machines that
I upgraded with dnf from fc26.

I noticed this first on the machines I upgraded because I usually follow the upgrades with some cleanups that include the following:

# dnf groups mark remove \*   # does not work properly on DNF-2
# So we have to mark the groups one by one 
dnf groups mark remove minimal-environment
dnf groups mark remove custom-environment
dnf groups mark remove base-x
dnf groups mark remove mate-desktop-environment
dnf groups mark remove mate-applications
dnf groups mark remove mate-compiz
dnf groups mark remove core

# Install whatever is missing and implicitly mark packages as installed
# with the group they belog to.
dnf -y install @minimal-environment @custom-environment @base-x  \
               @mate-desktop-environment @mate-applications      \
               caja-beesu                                        \
               --exclude mate-menus-preferences-category-menu    \
               --exclude caja-terminal                           \
               --exclude cinna\*  --exclude @cinna\*             \
               --exclude kde\* --exclude kf5\*                   \
               --allowerasing --exclude kernel\*


Under fc27 the latter takes *several* minutes just to resolve the packages list, while with f25 (on the same hardware) it consistently does it in a matter of seconds.


====================== Execution Under fc25 ============================

# export LANG=C ; time dnf -y install @... (see above for the full command)

Last metadata expiration check: 3:50:21 ago on Sun Oct  8 09:30:22 2017.
Environment 'Minimal Install' is already installed, skipping.
Environment 'Fedora Custom Operating System' is already installed, skipping.
Group 'base-x' is already installed, skipping.
Environment 'MATE Desktop' is already installed, skipping.
Group 'MATE Applications' is already installed, skipping.
Package caja-beesu-1.18.1-1.fc25.x86_64 is already installed, skipping.
Dependencies resolved.
Nothing to do.
Complete!

real	0m4.855s
user	0m4.015s
sys	0m0.209s


====================== Execution Under fc27 ============================

# export LANG=C ; time dnf -y install @... (see above for the full command)

Last metadata expiration check: 0:19:58 ago on Sun Oct  8 14:11:47 2017.
Environment 'Minimal Install' is already installed.
Group 'Core' is already installed.
Environment 'Fedora Custom Operating System' is already installed.
Group 'Core' is already installed.
Group 'base-x' is already installed.
Environment 'MATE Desktop' is already installed.
Group 'base-x' is already installed.
Group 'Standard' is already installed.
Group 'Core' is already installed.
Group 'Administration Tools' is already installed.
Group 'Dial-up Networking Support' is already installed.
Group 'Fonts' is already installed.
Group 'Input Methods' is already installed.
Group 'Multimedia' is already installed.
Group 'Common NetworkManager Submodules' is already installed.
Group 'Hardware Support' is already installed.
Group 'Printing Support' is already installed.
Group 'Guest Desktop Agents' is already installed.
Group 'MATE' is already installed.
Group 'MATE Applications' is already installed.
Package caja-beesu-1.18.1-4.fc27.x86_64 is already installed, skipping.
No match for group package "gstreamer1-plugins-bad"
No match for group package "rfremix-logos"
No match for group package "ubuntu-font-family"
No match for group package "caja-terminal"
No match for group package "xorg-x11-drv-omap"
No match for group package "workaround-cyrillic-console"
No match for group package "mate-menus-preferences-category-menu"
No match for group package "kmod-samsung-backlight"
No match for group package "xorg-x11-drv-armsoc"
No match for group package "kmod-rt3090"
No match for group package "rfremix-release"
No match for group package "kmod-rt3062"
No match for group package "kmod-brcm80211"
No match for group package "madwimax"
No match for group package "kernel-modules-extra"
Dependencies resolved.
Nothing to do.
Complete!

real    15m7.784s
user    14m51.222s
sys     0m1.498s