3.5.h [i] Describe fast convergence requirements
EIGRP has been designed and used to achieve sub-second convergence for years. Lab testing has shown that the key factor for EIGRP convergence is the presence or absence of a feasible successor. When there is no feasible successor, EIGRP uses queries to EIGRP peers and has to wait for responses. This slows convergence.
Proper network design is required for EIGRP to achieve fast convergence. Summarization helps limit the scope of EIGRP queries, indirectly speeding convergence. Summarization also shrinks the number of entries in the routing table, which speeds up various CPU operations. The effect of CPU operation on convergence is much less significant than the presence or absence of a feasible successor. A recommended way to ensure that a feasible successor is present is to use equal-cost routing.
EIGRP metrics can be tuned using the delay parameter. However, adjusting the delay on links consistently and tuning variance are next to impossible to do well at any scale. In general, it is unwise to have a large number of EIGRP peers. Under worst-case conditions, router CPU or other limiting factors might delay routing protocol convergence.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3248-3252). . Kindle Edition.