On my Vista Ultimate SP2 x64 it jumped from 2440 to 6076 but not being familiar with the process I wouldn't be able to interpret why it's doing that and especially more so on your rig. Technical Support may be able to advise better on that process - see the link under Useful links at the top of this page. I would imagine it all depends on what is using the process in the first place. I do know that it can be the sign of infection if it displays a constantly high number. It remained at the higher figure after I stopped the scan - probably will until the next reboot. However I noticed that at no time was it using any CPU resources, only memory.
To be on the safe side try scanning with the FREE version of THIS tool and update it before running. It also can do all that in Safe Mode with Networking if ever an infection disables it in regular mode. As you may have it already installed, simply update it and run.
Never run two scans of any kind together. Apart from the possibility of overheating ergo damage to the machine, they could feasibly clash with each other and lose their effectiveness as a result.
Well, for the lsass.exe it doesn't use any CPU either for my machine. Basically if McAfee isn't running a full scan or scheduled scan, it will always stay at some constant number between 3000-4000. But as soon as the scans start it rockets up to the 35000+. Like today it stayed at 37,916 K the whole scan and a little after the scan.
As.for running two scanners. That was not my intention, since I was trying to get the Malwarebytes scan to finish before 2 hours after I started up my computer has passed. Since I read that the new McAfee won't do a scheduled scan if there is mouse, keyboard activity and if the CPU isn't idling. But my McAfee will do the schedule scan 2 hours aftef my computer has started regardless is Malwarebytes was using 100% of my CPU. But basically it took about 2 minutes to cancel the McAfee scan since the GUI was lagging due to McAfee and Malwarebytes.
Have there been any reports about if Malwarebytes was in the last step of its full scan and McAfee scheduled scan started, it affected what Malwarebytes found or missed? Since my Malwarebytes showed two PUM, which is the usually ones that say disable.antivirus and disable.firewall. Since I never added these to the ignore list, I would get these every scan and I just choose not to ignore or remove.
It probably didn't have any effect on McAfee but one never knows. Those PUM's are a common finding on XP for some reason and are not important. Ignore those. On my machine with its high resources it probably wouldn't matter two scans overlapping but with an XP machine, which obviously has less memory it might.