Try even more things to address fetching narinfo slowness

guix-data-service/model/nar.scm

@@ -386,7 +386,7 @@ ORDER BY COUNT(*) DESC")
           #:key
           build-success-after
           after-id
-          (limit 500))
+          (limit 200))
   (define query
     (string-append
      "

guix-data-service/substitutes.scm

@@ -81,6 +81,10 @@
   (let loop ((last-id #f)
              (requests 0)
              (success-responses 0))
+    ;; I think something here is causing lots of garbage or garbage collection
+    ;; to be slow, so try and take breaks
+    (unless specific-outputs
+      (sleep 10))
     (let ((outputs-chunk
            (or specific-outputs
                (select-outputs-without-known-nar-entries