The Nexus 5 came out late 2013, the iPhone 4 in mid 2011, so a little over 2 years. Also I believe newer devices tend to be less snappy, as the tendencies for grand flourishes of OS animations and hardware performance optimization seems to be moving at different paces.

I find the Nexus 5 snappier than every other Android device I have used since then including powerhouses like the 5X, 6P, Note 7, and snappier than many modern iPhones I've used in my dev shop work (5S onwards, haven't tried the X).

It's only technical downside is a lower max screen brightness than other devices, but I personally prefer my screens not overpowering my eyes anyway.