If you have such strict time limitation for retrospective, then lean coffee format is that you need (by my opinion, of course):
1) Select a theme (optional) [note: possible improvements for current process, for example]
2) Everyone writes their name on a sticky and keeps it in front of them. (Again, it’s optional, but name cards are very appreciated if not everyone knows eachother.) [note: no need for your team, of course]
3) Each person writes topics onto notecards (one topic per card). No limit to the number of topics per participant. (Pro-tip: keep the words per card to a minimum for readability.) [note: In your case each person should write a one possible improvement for current process.]
4) Set up a personal kanban board using three sticky notes, one for each column: To Do, Doing, Done (or To Discuss, Discussing, Discussed) from left to right. (Pro-tip: add a fourth column (“Actions”) to collect action items as they come up.)
5) Spend a few minutes (2-5) introducing each topic. Every author should share a sentence or two describing the idea they wrote on a given card. This adds a bit more detail and insight.
6) Vote. Each participant is given two votes (or three, depending on the size of your group). Simple dot-voting is quick and effective. Note that you may cast all your votes on one topic or spread them across multiple cards.
7) Prioritize the cards based on the voting results. Rank the more popular topics higher in the backlog (the “To Do” column).
8) Now that you have a democratic agenda, move the top item into the middle (“Doing”) column.
9) Set the timer (typically on a smartphone) for five minutes — or whatever length the group determines is reasonable. (Maybe eight is a better length; it depends on your group size & preferences.) This is the initial timebox for discussion.
10) When the time limit is reached, hold a simple roman vote (thumbs up, sideways or thumbs down) to see if there’s interest in continuing the discussion. If so, set the timer for a shorter duration (eg. three minutes) and continue discussing. You can repeat this step as many times as necessary until the group loses interest in the topic.
11) When the topic runs out of gas, move its card to the right (“Done”) column. Bring the next highest card over from “To Do” into “Doing” and repeat the process.
12) Optionally, at the end of the session, go around and elicit key take-aways and/or action items from the group. This is usually important if you’re using the meeting to drive decisions or create work. Taking a photo of the board or any artifacts (mind maps, etc.) is also handy.
Of course, even in this short format 20 minutes is not enough to take coordinated decision how to improve working process in your team and make a plan how to implement these improves. But in any case you will detect a most popular suggestion for process improvements (by common opinion) and you will have time to discuss some of them. It is better than nothing.