diff --git a/lib/irc/ClientPool.js b/lib/irc/ClientPool.js
index 7314b2e32..923e7c84e 100644
--- a/lib/irc/ClientPool.js
+++ b/lib/irc/ClientPool.js
@@ -28,6 +28,13 @@ function ClientPool(ircBridge) {
         //    }
         // }
     }
+    // map of numbers of connected clients on each server
+    // Counting these is quite expensive because we have to
+    // ignore entries where the value is undefined. Instead,
+    // just keep track of how many we have.
+    this._virtualClientCounts = {
+        // server_domain: number
+    };
 }
 
 ClientPool.prototype.killAllClients = function() {
@@ -77,6 +84,7 @@ ClientPool.prototype.createIrcClient = function(ircClientConfig, matrixUser, isB
             nicks: Object.create(null),
             userIds: Object.create(null)
         };
+        this._virtualClientCounts[server.domain] = 0;
     }
     if (isBot) {
         this._botClients[server.domain] = bridgedClient;
@@ -93,6 +101,7 @@ ClientPool.prototype.createIrcClient = function(ircClientConfig, matrixUser, isB
     // connected yet, else we could spawn 2 clients for a single user if this
     // function is called quickly.
     this._virtualClients[server.domain].userIds[bridgedClient.userId] = bridgedClient;
+    this._virtualClientCounts[server.domain] = this._virtualClientCounts[server.domain] + 1;
 
     // Does this server have a max clients limit? If so, check if the limit is
     // reached and start cycling based on oldest time.
@@ -198,14 +207,7 @@ ClientPool.prototype._checkClientLimit = function(server) {
 
 ClientPool.prototype._getNumberOfConnections = function(server) {
     if (!server || !this._virtualClients[server.domain]) { return 0; }
-
-    var connectedNickMap = this._virtualClients[server.domain].nicks;
-
-    var numConnectedNicks = Object.keys(connectedNickMap).filter(function(nick) {
-        return Boolean(connectedNickMap[nick]); // remove 'undefined' values
-    }).length;
-
-    return numConnectedNicks;
+    return this._virtualClientCounts[server.domain];
 };
 
 ClientPool.prototype.countTotalConnections = function() {
@@ -227,6 +229,7 @@ ClientPool.prototype._removeBridgedClient = function(bridgedClient) {
     var server = bridgedClient.server;
     this._virtualClients[server.domain].userIds[bridgedClient.userId] = undefined;
     this._virtualClients[server.domain].nicks[bridgedClient.nick] = undefined;
+    this._virtualClientCounts[server.domain] = this._virtualClientCounts[server.domain] - 1;
 
     if (bridgedClient.isBot) {
         this._botClients[server.domain] = undefined;