Number of trigger campaigns: Trigger campaigns are always on, always listening. If there are 50 campaigns triggering at the same time, all the 50 triggers will be in queue, slowing down your processing and routing inside Marketo.
Complexity of smart lists: The more complex a smart list, the harder it is for the system to figure it out, which increases backend processing and even creates campaign failures from timeouts.
Volume of Leads: With regular cleaning and good systems design, it is fairly easy to keep your system running fast. Reduce the number of leads that can flow through with filters. Clean up the inactive leads at regular intervals.