Xcode 10 Storyboard extremely slow to the point of unresponsive

I had extremely slow only when editing on xib file in Xcode version 10.1. What worked for me was to disable the Source Control under Xcode/Preferences/Source Control and then I checked "Enable Source Control" again and it fixed it.

I found the anwser to my own question for anyone who is having this issue.

1. clear your xcuserdata and xcworkspace from your Xcode project by clicking show package contents on your xcodeproj file. Move xcuserdata and xcworkspace to the trash.

2. This allows you to open up your Xcode project again and be able to open storyboard as source code. It resets the last file you had open. To do this right-click (or two-finger click on track-pad) on main.storyboard and choose Open as Source Code

3. At the top you'll see some code similar or identical to this

<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="12121" systemVersion="16G29" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" colorMatched="YES" initialViewController="OjV-iF-7f7">
<device id="retina5_5" orientation="portrait">
    <adaptation id="fullscreen"/>
</device>
<dependencies>
    <deployment identifier="iOS"/>
    <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="12089"/>
/* delete this here */
<capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
</dependencies>
<customFonts key="customFonts">
    <array key="small_pixel.ttf">
        <string>Small-Pixel</string>
    </array>
</customFonts>
<scenes>

Don't panic, you are only looking for one line. No editing needs to be done.

<capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/"  

4. Once you've found it, delete the entire line from the code.

You should now be able to open up the main.storyboard file as the normal interface builder by right-clicking (two-finger click on track-pad) and choosing Open as Interface Builder

(01/07/2020) Reports state this still works for the latest version of Xcode.