I started to experience this on Beta 049 and now in 1.0.0 & 1.0.1 - I am unable to work out why, in terms of memory usage nor can I blame it on the auto-save. Or is it just little old me ? 
049 and the beta before it did have some severe lag problems, but the main bugs related to those were fixed for 1.0, so if you uninstalled 049 and did a clean install of 1.0 on your machine, this might be a new issue or it might just be a large project on a slower maching bumping up against the current performance limitations of the code framework. Leeās going to be rewriting much of the text display code over the next couple months to work around these issues, so performance will significantly improve, but even so you shouldnāt be having major issues now. How large is the project youāre working in? Do you see the lag everywhere or just when working in a lengthy Scrivenings session? What are your machine specsāRAM, processor speed, etc.? More detailed info here would help track down the issue.
More detailed info I am unable to provide. You can see the system(s) I am using Scrivener on in my signature.
There is nothing untoward that I can see in the Windows Task Manager to account for any leakage in memory that could account for the lag.
The lag, in itself, isnāt a long one - but it is noticeable.
I did not clean sweep my system before I loaded 1.0.0, nor 1.0.1 so I shall give that a try, although my Control Panel says I have only one version of Scrivener loaded, and that is this latest one.
One project is quite small 121kb, another is large but by no means āmassiveā - 1880kb - both currently on my PC; BUT I am working on a flash drive (but then I have since late spring this year), with little trouble until the last beta and now 1.0.1.