--- a/models/wekanCreator.js
+++ b/models/wekanCreator.js
@@ -323,11 +323,13 @@ class WekanCreator {
     // now add other members
     if (boardToImport.members) {
-      boardToImport.members.forEach(wekanMember => {
-        // is it defined and do we already have it in our list?
-        if (
-          wekanMember.wekanId &&
-          !boardToCreate.members.some(
-            member => member.wekanId === wekanMember.wekanId,
-          )
-        )
-          boardToCreate.members.push({
-            ...wekanMember,
-            userId: wekanMember.wekanId,
-          });
-      });
+      // Build a Set of already-present wekanIds so dedup is O(1) per member
+      // instead of O(M) per member (was O(M^2) overall).
+      const seenWekanIds = new Set(
+        boardToCreate.members.map(m => m.wekanId).filter(Boolean),
+      );
+      boardToImport.members.forEach(wekanMember => {
+        if (wekanMember.wekanId && !seenWekanIds.has(wekanMember.wekanId)) {
+          seenWekanIds.add(wekanMember.wekanId);
+          boardToCreate.members.push({
+            ...wekanMember,
+            userId: wekanMember.wekanId,
+          });
+        }
+      });
     }

@@ -410,18 +412,22 @@ class WekanCreator {
       // add members
       if (card.members) {
-        const wekanMembers = [];
-        // we can't just map, as some members may not have been mapped
-        card.members.forEach(sourceMemberId => {
-          if (this.members[sourceMemberId]) {
-            const wekanId = this.members[sourceMemberId];
-            // we may map multiple Wekan members to the same wekan user
-            // in which case we risk adding the same user multiple times
-            if (!wekanMembers.find(wId => wId === wekanId)) {
-              wekanMembers.push(wekanId);
-            }
-          }
-          return true;
-        });
+        // Use a Set for O(1) dedup instead of Array.find() O(M) per member.
+        const seenMemberIds = new Set();
+        const wekanMembers = [];
+        card.members.forEach(sourceMemberId => {
+          if (this.members[sourceMemberId]) {
+            const wekanId = this.members[sourceMemberId];
+            if (!seenMemberIds.has(wekanId)) {
+              seenMemberIds.add(wekanId);
+              wekanMembers.push(wekanId);
+            }
+          }
+          return true;
+        });
         if (wekanMembers.length > 0) {
           cardToCreate.members = wekanMembers;
         }
@@ -429,18 +435,22 @@ class WekanCreator {
       // add assignees
       if (card.assignees) {
-        const wekanAssignees = [];
-        // we can't just map, as some members may not have been mapped
-        card.assignees.forEach(sourceMemberId => {
-          if (this.members[sourceMemberId]) {
-            const wekanId = this.members[sourceMemberId];
-            // we may map multiple Wekan members to the same wekan user
-            // in which case we risk adding the same user multiple times
-            if (!wekanAssignees.find(wId => wId === wekanId)) {
-              wekanAssignees.push(wekanId);
-            }
-          }
-          return true;
-        });
+        // Same Set-based dedup for assignees.
+        const seenAssigneeIds = new Set();
+        const wekanAssignees = [];
+        card.assignees.forEach(sourceMemberId => {
+          if (this.members[sourceMemberId]) {
+            const wekanId = this.members[sourceMemberId];
+            if (!seenAssigneeIds.has(wekanId)) {
+              seenAssigneeIds.add(wekanId);
+              wekanAssignees.push(wekanId);
+            }
+          }
+          return true;
+        });
         if (wekanAssignees.length > 0) {
           cardToCreate.assignees = wekanAssignees;
         }