What ROM version are you using?
Re the Lag: Samsung have been working on improving the filesystem for some time. At one stage it was believed it could have gone into the Froyo upgrade but was found to be impossible to maintain.
Although they are still working on it (info as of Sat 20th Nov) I do know there were issues with the new phones that are coming in and part of the support was pulled temporary.
There's no firm date but I'm still hopeful we will see an announcement in the next month.
As a short term fix, if the lag is that bad a can thoroughly recommend you use RyanZA's
z4root and RyanZA's
OCLF.
I have made several posts on these matters but RyanZA's tools make root & lagfix such a simple one-click affair.
There is no need for a PC, no need for any third-party software to install on a PC (Odin). All you need is a copy of both files and away you go.... rooted and lag fixed easy as that.
Both tools are now also available on the Market but I would first have a read on the two links I have provided so you can be fully in the picture how easy and safe it is.
Re GPS: This has become rather strange of late.....
Prior to the Froyo my GPS was totally horrendous. I couldn't get a lock for several minutes and when I did my location bounced all over the place, never maintaining an accurate lock and fix.
However, since applying the JPO ROM upgrade my SGS is now superb in all matters including GPS. Fore the very first time I get a lock in seconds and note I'm actually seeing and picking up satellites with SNR's below 20 which was totally unheard of on the SGS. Not only that but my position is always spot on pinpoint accurate. I simply now can not fault GPS and it's not drifted once since doing the upgrade coming two weeks now.
I'd like to know more on why some SGS devices are great whilst others remain poor. This however is not the right thread but I'd be happy to discuss it further with you in the relevant GPS topics.
