diff --git a/leaderboard/contributors/models.py b/leaderboard/contributors/models.py
index 57e1a04..f84ceff 100644
--- a/leaderboard/contributors/models.py
+++ b/leaderboard/contributors/models.py
@@ -1,3 +1,4 @@
+from collections import defaultdict
 import operator
 
 from bulk_update.helper import bulk_update
@@ -67,28 +68,48 @@ def _compute_ranks(cls, contributions):
             (rank.contributor_id, rank.country_id): rank
             for rank in ContributorRank.objects.all()
         }
-        countries = Country.objects.all()
+
+        contributor_prior_countries = defaultdict(set)
+        for contributor_id, country_id in contributor_ranks.keys():
+            if country_id is not None:
+                contributor_prior_countries[contributor_id].add(country_id)
+
+        countries = (Country.objects.order_by('-area')
+                                    .only('id', 'geometry').all())
+        prepared_countries = dict([(country.id, country.geometry.prepared)
+                                   for country in countries])
 
         for contribution in contributions:
             # Each contribution counts towards the rank in the country in which
             # it was made, as well as the global rank for that contributor.
 
-            contribution_country = None
+            contribution_country_id = None
 
-            # Attempt to find a country which contains the observation point
-            for country in countries:
-                if country.geometry.contains(contribution.point):
-                    contribution_country = country
+            # Check if the contribution is in one of the countries the
+            # contributor has contributed to before.
+            for country_id in contributor_prior_countries.get(
+                    contribution.contributor_id, ()):
+
+                prepared = prepared_countries.get(country_id)
+                if prepared and prepared.contains(contribution.point):
+                    contribution_country_id = country_id
                     break
 
+            # Attempt to find a country which contains the observation point
+            if contribution_country_id is None:
+                for country_id, prepared in prepared_countries.items():
+                    if prepared.contains(contribution.point):
+                        contribution_country_id = country_id
+                        break
+
             # If no point was found, find the nearest country
-            if contribution_country is None:
-                contribution_country = sorted([
+            if contribution_country_id is None:
+                contribution_country_id = sorted([
                     (country.geometry.distance(contribution.point), country)
                     for country in countries
-                ])[0][1]
+                ])[0][1].id
 
-            for country_id in (contribution_country.id, None):
+            for country_id in (contribution_country_id, None):
                 rank_key = (contribution.contributor_id, country_id)
                 contributor_rank = contributor_ranks.get(rank_key, None)