Explain better

apollo-router/src/apollo_router.rs

@@ -50,10 +50,10 @@ impl ApolloRouter {
         // We don't need to do this in background because the old server will keep running until
         // this one is ready.
         //
-        // We might lose the benefit of warming the cache up, since it could immediately get some
-        // queries that could have been precalculated. If we block on warming up, starting the
-        // router is slightly slower, but when reloading configuration there's no delay, we create
-        // the router before restarting the server
+        // If we first warm up the cache in foreground, then switch to the new config, the next
+        // queries will benefit from the warmed up cache. While if we switch and warm up in
+        // background, the next queries might be blocked until the cache is primed, so there'll be
+        // a perf hit.
         if let Some(previous_router) = previous_router {
             for (query, operation, options) in previous_router.query_planner.get_hot_keys().await {
                 // We can ignore errors because some of the queries that were previously in the