How to optimize merging of lists?

It's incredibly slow on a server with dual processors and 64gb of ram and an SSD.

It takes 5 mins on my home PC but then again importing takes forever with the system as well.

Any help appreciated.
It's taking a lot because it also does a lot of checks in the database, for duplicates but also it checks for email address validity and many other things, which in the end might give a slow speed.
What version are you currently on?
Have you tried doing an update to latest version( as speaking) and see if that improves things?
How many subscribers the system has, how about campaigns and lists and delivery logs?
In order to make give a break to mysql, maybe try removing old campaigns, this way the database size will lower and the sql queries will b faster.

I am just comparing the speed to other mailing software I use(that doesnt connect to ESPs - hence using Mailwizz for SES and Mandrill) . I am not on actually, has that had optimizations ? The system has around 1,7 million subscribers (244,646 unique) but I have a lot of lists (66) because my data feeds are daily.

I will clear some campaigns and check if it speeds everything up.