What settings are you using? There are many settings that will increase your load, especially "activity today" settings.
Have you tried enabling the mod at a low activity time and seeing how it reacts when activity rises? (because if you have a lot of users, activity must be calculated as posts are viewed authored by users whose activity hasn't been calculated yet for the day; you can "schedule" this load increase to happen when load in general is low) If this is effective, you can set the "midnight offset" setting to keep this schedule. (changing the setting, however, invalidates all activity calculated to that point since the setting effects the calculation)
A
load average of 5.00 indicates 100% utilization (for a 1 to 5 core server), so 55% would be a load decrease...
When you say the old version for the older vbulletin, you are refering to vB3.0 or vB3.5?