Here is the proof that raise is slower than if

require 'benchmark'
n = 5000000

puts "convertions"
Benchmark.bm do |x|
 x.report("rescues") { n.times do   ; true rescue ''; end }
 x.report("ifs") { n.times do   ; raise '' if false; end }
end

The results

 
user     system      total        real
rescues  0.880000   0.000000   0.880000 (  0.880214)
ifs      0.790000   0.000000   0.790000 (  0.791239)

You win 0.09 seconds per 5m requests.
If you serve one page for 0.20 sec you have to made apx 10m page views and you will have one free as a reward of using ifs

But did you win? How ugly is your code?