Avoid copying json::Value objects a lot when reallocating vector

Author: danpat

CHANGELOG.md

@@ -1,3 +1,8 @@
+# 5.16.3
+  - Changes from 5.16.2:
+    - Performance
+      - FIXED: Speed up response time when lots of legs exist and geojson is used with `steps=true` [#4936](https://github.com/Project-OSRM/osrm-backend/pull/4936)
+
 # 5.16.2
   - Changes from 5.16.1:
     - Bugfixes:

include/engine/api/route_api.hpp

@@ -211,12 +211,16 @@ class RouteAPI : public BaseAPI
         }
 
         std::vector<util::json::Value> step_geometries;
+        const auto total_step_count =
+            std::accumulate(legs.begin(), legs.end(), 0, [](const auto &v, const auto &leg) {
+                return v + leg.steps.size();
+            });
+        step_geometries.reserve(total_step_count);
+
         for (const auto idx : util::irange<std::size_t>(0UL, legs.size()))
         {
             auto &leg_geometry = leg_geometries[idx];
 
-            step_geometries.reserve(step_geometries.size() + legs[idx].steps.size());
-
             std::transform(
                 legs[idx].steps.begin(),
                 legs[idx].steps.end(),