A micro sd card is always good to have IMO. You can store music and such on it and it will play without problems. And like said above you can install apps to the sd card... well partially. Only part's of apps can be moved to external storage and they won't load first thing since the micro sd card has to load first.
Are micro sd cards slower than internal storage? Yes significantly depending on the class of the card where class 2 being the slowest and going up by 2's till you get to class 10 where that is the fastest. So there is class 2, 4, 6, 8, and 10.
Generally you won't notice the speed difference between them unless you are running a rom off of the sd card (via bootmanager) or transferring files to it. Also you need a class 6 or higher card to be able to record HD video without stutter.
A 64gb micro sdcard once formatted on the phone will perform very well. And you should be able to record HD video without any problems.
TL;DR version: It won't function
completely like internal storage and will be a bit slower, but it shouldn't have any effect on a day to day basis, although you may notice that it is slower writing data to the micro sdcard than internal storage. Generally I only use a micro sdcard for music and such, but if you feel the need to install apps, well partial parts that can be moved to external storage, you should have no problem whatsoever.
Hope this helps
