This has been an issue for a very long time. I've used Site Designer on many different machines and it is consistent across all devices.

However with the current release (5.0.3528) is appears to have become worse.

If I open a project and maybe edit some text or perform minor changes, leave it open overnight and come back to it in the morning then SD has slowed to a crawl. It takes about 3 minutes to save and exit the program and when restarted it operates at normal responsive speed. Over time it will slow again until it becomes too difficult to use and requires a close and restart to bring it back to speed again.

I'm guessing that there is some kind of dynamic undo buffer/cache/database or something that gets progressively slower over time of operation but is cleaned out on a restart.
All other software on the machine is operating normally and does not slow down so it the problem appears to be SD itself.

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Although I must also say, that if I work on a project for more than 2 or 3 hours at a time, yes it gets slow, 4-5 hours almost stops, but a simple save, then wait 2 mins open again I'm good to go for another couple of hours.

I have raised this lagging issue with the CC team, but so far they don't seem to be able to find what's causing it.

Empirical evidence so far seems to indicate that it the slow down will appear some time later (could be a few hours, or overnight) after text edits are made. This is what leads me to suggest that there's some kind of undo buffer/data store that is leaking memory and causing internal speed issues.

It's a bit annoying to be working on a project; do some graphic edits in another program and then come back to work with those graphics to find SD has become sluggish and requires restarting to continue effectively.