require 'ruby-prof' # you don't need this if you have ruby-prof in your Gemfile

You can set one of the things you want to measure by using (default is PROCESS_TIME most useful ones are PROCESS_TIME, MEMORY, CPU_TIME):
RubyProf.measure_mode = RubyProf::PROCESS_TIME
RubyProf.measure_mode = RubyProf::WALL_TIME
RubyProf.measure_mode = RubyProf::CPU_TIME
RubyProf.measure_mode = RubyProf::ALLOCATIONS
RubyProf.measure_mode = RubyProf::MEMORY
RubyProf.measure_mode = RubyProf::GC_RUNS
RubyProf.measure_mode = ...

Wether you modify an existing named scope or add a new one, or when you write a new query, make sure you have the proper indices.

This particularly applies if you're going to run non-trivial queries of course (admin backends, analytics, etc).

Compound indices

A chain of scopes results in (usually) one query. You should take into account all attributes (columns) that are used in :conditions, :join, :group, :having, and :order, as all those result in filtering and sorting–slow operations without indices.

Take the list of all ...

Regularly, but at least before merging your feature branches, rebase them to get rid of "fix typo" and "wip" commits.

Getting rid of unnecessary commits

Let's say you do a git rebase -i HEAD~~~~ and have this commit history:

pick 1d1e1f My Feature
pick 2d2e2f My Feature - wip
pick 3d3e3f My Feature - fix typo
pick 4d4e4f My Other feature

Change it to use fixup for those commits that should be merged into the one before ...