CEF is supported for 7000, 7200 routers and above with certain memory requirements. However, I think you still can enable cef on a 2620 router if you have the right IOS image, but in that case, the IOS will select the next fastest switching path like optimum, process switching or fast switching.
CEF is not CPU intensive, but takes a lot of memory though. CEF if implemented properly, it can lower your CPU utilization.
Check the following link for more information on CEF,
CEF can actually save system resources compare to that of other switching mechanisms (i.e FastSwitching) but it is good to read about before enabling to ensure it will give you want you want. Also it helps when troubleshooting CEF related problems.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...