[rdtsc]

Others mentioned Observer. That is very cool.

When without a GUI I use the built-in dbg module. Sometimes via the recon_trace wrapper to get the tracing to be rate limited (if on a production cluster).

ttb built-in modules used for tracing on multiple nodes, capture more data and then save to a file and inspect later.

eflame or eep (not the Erlang Proposal, the profiling tool) + kcachegrind to view results.