Okay, I want you to get to the bottom of this, so I'm going to post a screen shot of what this sluggish behavior looks like in my environment.
First, this is build 9.0.0.97
I lost this entire post just a few minutes ago, so I'm rather pissy right now and will be far more brief than I was in the first go at it. Sorry.
To do this test, I launched WebBuilder and started a new html document. I had a line of code I was messing with from a previous editing session in my clipboard manager, so I just pasted it in.
The sluggishness I experience is in keyboard navigating around in code. So, for instance Ctrl+Shift+Arrow to select text a "word" at a time. Or Shift+Arrow to select character by character. Or Ctrl+Arrow to move a word at a time. And yes, even just an arrow key to move character by character without selecting anything.
BUT - AND THIS MAY BE SIGNIFICANT:
All of the above involve single taps on the arrow keys. If I hold down the arrow key to engage the auto repeat, then the cursor/selection zips right along.
There is little precision in moving at the auto repeat rate, however, and I'm sure everyone would agree that single arrow taps are what we use to select stuff, or move with precision to a specific place.
But with single arrow taps, the cursor motion is so sluggish that movement actually leaves a cursor echo on the line I'm moving through, and when selecting code, the selection lags even behind the cursor echo. In the screen shot below, I was positioned where the selection highlight begins, then held down the Ctrl+Shift and tapped the RightArrow about 8 times. It's so sluggish that I had absolutely no trouble letting go of all keys and tapping the Print Screen key to capture the effect. I'm giving you the Full Monty screen shot so you can see the entire WebBuilder environment.
Karlis, I'm sure you'll have questions at this point.
This behavior began with WebBuilder 2008. I also saw this same phenomenon in Evrsoft's First Page 2006 which also introduced begin/end tag highlighting, so I suspected that as the culprit. I have turned that off, however, with no change in the symptoms. So, naturally I turned it back on. It's an awesome feature that has already pointed out some goofs I've made.
Here's the screen shot:
Hope this has been helpful, and as I say, please don't hesitate to be in touch if you need more info. I really want to see you get to the bottom of this.