index: Lower parallel load to 4

internal/repository/repository.go

@@ -369,7 +369,7 @@ func (r *Repository) SaveFullIndex(ctx context.Context) error {
 	return r.saveIndex(ctx, r.idx.FullIndexes()...)
 }
 
-const loadIndexParallelism = 20
+const loadIndexParallelism = 4
 
 // LoadIndex loads all index files from the backend in parallel and stores them
 // in the master index. The first error that occurred is returned.