Bit of an odd posts, asking about a text based installer were you do the partitioning in cli, yet no reading of any on that subject seems done..........
If you had opened the Installation Guide in Live mode, you would have seen the part of using GPT, and seen the link how to do that the recommended cli way -->
Why would you think cli apps available in a text installer wouldn't be available in GUI? Anything you would use in such an installer is available, gptfdisk, parted, cgdisk, etc. Plus there is the option to install gparted in live mode, in case you don't like kpartitionmanger and want to work with a GUI.
Next part, there are many posts here, in the News section, G+ channel, about how this Installer was temporary. Latest News even posts a preview video of the new installer that is worked on.
Simple search of these forums shows quite a few discussions about needing a new installer, example:
And there are news items/posts mentioning the cooperation between many distros to get an universal installer, example:
About your system being slow from USB, with 4 Gb of RAM, best to take a look at the used USB stick first, If that one reads very slow, then ofc the system is slow.