"Reverse Groups" challenge
I can't say what CodeEval's time limits are, or why this code should particularly slow. There are a lot of things you could do differently, though:


Instead of readlines, you can use each_line to iterate the lines, instead of reading all of them into an array, and then iterating.
You can get the values and k in a single line:  

*values, k = line.chomp.split(/[,;]/)

Look up each_slice; it'll iterate an array in "chunks" of the given length
Don't do "manual" string concatenation. You already use join in one place, but you can use it anywhere you need a string representation of an array.
You're shifting k items off of the array, and you then check if the number of shifted items is divisible by k. But shift will either shift exactly k items or fewer, so the modulus operator is a really roundabout way of saying current.count == k.
You code fails if there are blank lines. CodeEval doesn't say whether the file can contain blank lines, but, if it does, well...


All in all, you're approaching this very procedurally. A nicer approach would be something like


Chunk the items in k-sized arrays (the last chunk may be smaller)
Reverse the chunks that are k items long
Put the arrays back together into one
Join with commas and print


Or, in code:

File.open(ARGV.first).each_line do |line|
  *items, k = line.chomp.split(/[,;]/)
  next unless k # skip empty lines
  puts items.each_slice(k.to_i).map { |group| group.count &lt; k.to_i ? group : group.reverse }.flatten.join(",")
end


Edit: As Naklion points out in the comments, using flat_map is of course better than using map and flatten separately, i.e.:

items.each_slice(k.to_i).flat_map { |group| group.count &lt; k.to_i ? group : group.reverse }.join(",")


I've update the rest of the code blocks to use flat_map

That 3rd line is a bit long, though, so to spell it out:

File.open(ARGV.first).each_line do |line|
  *items, k = line.chomp.split(/[,;]/)
  next unless k
  chunks = items.each_slice(k.to_i)
  reversed = chunks.flat_map { |chunk| chunk.count &lt; k.to_i ? chunk : chunk.reverse }
  puts reversed.join(",")
end


You can also do pop off the last chunk if you know it to be smaller than k, which lets you say simply map(&amp;:reverse) on the remaining elements

File.open(ARGV.first).each_line do |line|
  *items, k = line.chomp.split(/[,;]/)
  next unless k
  k = k.to_i
  chunks = items.each_slice(k).to_a
  tail = items.count % k == 0 ? [] : chunks.pop
  puts chunks.flat_map(&amp;:reverse).concat(tail).join(",")
end


Last alternative: Pop off enough items that the remaining ones are cleanly divisible by k before slicing

File.open(ARGV.first).each_line do |line|
  *items, k = line.chomp.split(/[,;]/)
  next unless k
  k = k.to_i
  tail = items.pop(items.count % k)
  puts items.each_slice(k).flat_map(&amp;:reverse).concat(tail).join(",")
end


But frankly, I like the first code block the best.

All of these are a little faster than your code, by the way. It's not a huge difference, but it's faster.