Hey Joel. I have found a fix, although not convenient, it has worked for me. I contacted Presonus in September about the same issue you all seem to be having. What they told me was that they were aware of the issue and that it definitely is being addressed but not sure when a fix will be released. That being said, I've tried it all, turned off plugins, messed with each possible CPU setting in the preferences and nothing would work. However, this past weekend I recorded a drum session that I wanted to quantize, 12 tracks of drums recorded at 24 bit, 44.1 - standard stuff.
I had split the work with one of my interns who runs Studio One on her MacBook pro ( way lower in spec than my 5K iMac with 32GB of RAM). I sent her the stems and she created a fresh session to do the quantizing. She used the same method I do, splice quantize for slip editing and she had zero lag time at all.
I tried this with the same song I was previously editing with lag only this time I started a fresh session and no lag time incurred whatsoever...if fact it runs perfectly smooth. Hope this also helps other users, there's gotta be some kind of temp file that continue to amass and slow the whole session down...idk hopefully soon there's a fix. Cheers!
Revision: Upon further investigating this issue, the lag seems to dramatically increase if the tracks are packed in a folder. Once you take them out, GUI performance seems to really improve!