The only thing with which I have lots of Linux experience is installation and two upgrades one with Red Hat Enterprise Linux 4 upgrading RHEL 3 and one with RHEL3 upgrading an older 2004 version of RH workstation.
The installations have always taken a while and just about as long as the upgrades. Depending on the hardware, the installations did not always go well.
For the most recent RHEL3 --> RHEL4 upgrade, the upgrade went well, and when my telnet/ssh login process times increased, Red Hat would not support it, because I had upgraded, rather than installed fresh.
I have not used MEPIS.