I'm using LMS 3.2 into ACS 4.0 and it actually seemed a bit faster after ACS integration. Nothing I measured but subjectively it seems faster. Both my servers are on Windows and the ACS is across the WAN from my CiscoWorks.
How do your devices fare with their ACS? You can debug tacacs at the router/switch level as one tool. I'm sure one of the cisco guys on here will point you to one of the many logs that LMS generates, possibly with debugging activated, to dig deeper there also.