diff --git a/tree/ntuple/v7/inc/ROOT/RNTupleDescriptor.hxx b/tree/ntuple/v7/inc/ROOT/RNTupleDescriptor.hxx
index ff2cb565a3825..bad70cbcffdc2 100644
--- a/tree/ntuple/v7/inc/ROOT/RNTupleDescriptor.hxx
+++ b/tree/ntuple/v7/inc/ROOT/RNTupleDescriptor.hxx
@@ -1405,6 +1405,8 @@ public:
    RResult<void> AddClusterGroup(RClusterGroupDescriptor &&clusterGroup);
    RResult<void> AddCluster(RClusterDescriptor &&clusterDesc);
 
+   RResult<void> AddNEntries(std::uint64_t nEntries);
+
    RResult<void> AddExtraTypeInfo(RExtraTypeInfoDescriptor &&extraTypeInfoDesc);
 
    /// Clears so-far stored clusters, fields, and columns and return to a pristine ntuple descriptor
diff --git a/tree/ntuple/v7/src/RNTupleDescriptor.cxx b/tree/ntuple/v7/src/RNTupleDescriptor.cxx
index c9f7ed49e4e29..51c471856f231 100644
--- a/tree/ntuple/v7/src/RNTupleDescriptor.cxx
+++ b/tree/ntuple/v7/src/RNTupleDescriptor.cxx
@@ -595,11 +595,9 @@ ROOT::Experimental::RNTupleDescriptor::AddClusterGroupDetails(DescriptorId_t clu
          return R__FAIL("invalid attempt to re-populate existing cluster");
       }
    }
-   std::sort(clusterIds.begin(), clusterIds.end(),
-             [this](DescriptorId_t a, DescriptorId_t b)
-             {
-                return fClusterDescriptors[a].GetFirstEntryIndex() < fClusterDescriptors[b].GetFirstEntryIndex();
-             });
+   std::sort(clusterIds.begin(), clusterIds.end(), [this](DescriptorId_t a, DescriptorId_t b) {
+      return fClusterDescriptors[a].GetFirstEntryIndex() < fClusterDescriptors[b].GetFirstEntryIndex();
+   });
    auto cgBuilder = Internal::RClusterGroupDescriptorBuilder::FromSummary(iter->second);
    cgBuilder.AddSortedClusters(clusterIds);
    iter->second = cgBuilder.MoveDescriptor().Unwrap();
@@ -949,10 +947,9 @@ ROOT::Experimental::RNTupleDescriptor ROOT::Experimental::Internal::RNTupleDescr
    for (const auto &[id, _] : fDescriptor.fClusterGroupDescriptors)
       fDescriptor.fSortedClusterGroupIds.emplace_back(id);
    std::sort(fDescriptor.fSortedClusterGroupIds.begin(), fDescriptor.fSortedClusterGroupIds.end(),
-             [this](DescriptorId_t a, DescriptorId_t b)
-             {
-               return fDescriptor.fClusterGroupDescriptors[a].GetMinEntry() <
-                      fDescriptor.fClusterGroupDescriptors[b].GetMinEntry();
+             [this](DescriptorId_t a, DescriptorId_t b) {
+                return fDescriptor.fClusterGroupDescriptors[a].GetMinEntry() <
+                       fDescriptor.fClusterGroupDescriptors[b].GetMinEntry();
              });
    RNTupleDescriptor result;
    std::swap(result, fDescriptor);
@@ -1173,6 +1170,12 @@ ROOT::Experimental::Internal::RNTupleDescriptorBuilder::AddClusterGroup(RCluster
    return RResult<void>::Success();
 }
 
+ROOT::RResult<void> ROOT::Experimental::Internal::RNTupleDescriptorBuilder::AddNEntries(std::uint64_t nEntries)
+{
+   fDescriptor.fNEntries += nEntries;
+   return RResult<void>::Success();
+}
+
 void ROOT::Experimental::Internal::RNTupleDescriptorBuilder::Reset()
 {
    fDescriptor.fName = "";
diff --git a/tree/ntuple/v7/src/RPageStorage.cxx b/tree/ntuple/v7/src/RPageStorage.cxx
index 3ad80586d8621..ebe156d32d6df 100644
--- a/tree/ntuple/v7/src/RPageStorage.cxx
+++ b/tree/ntuple/v7/src/RPageStorage.cxx
@@ -257,6 +257,7 @@ void ROOT::Experimental::Internal::RPageSource::UnzipClusterImpl(RCluster *clust
          continue;
       const auto &columnInfos = fActivePhysicalColumns.GetColumnInfos(columnId);
 
+      allElements.reserve(allElements.size() + columnInfos.size());
       for (const auto &info : columnInfos) {
          allElements.emplace_back(GenerateColumnElement(info.fElementId));
 
@@ -857,6 +858,8 @@ void ROOT::Experimental::Internal::RPagePersistentSink::UpdateSchema(const RNTup
    }
 
    const auto nColumns = descriptor.GetNPhysicalColumns();
+   fOpenColumnRanges.reserve(fOpenColumnRanges.size() + (nColumns - nColumnsBeforeUpdate));
+   fOpenPageRanges.reserve(fOpenPageRanges.size() + (nColumns - nColumnsBeforeUpdate));
    for (DescriptorId_t i = nColumnsBeforeUpdate; i < nColumns; ++i) {
       RClusterDescriptor::RColumnRange columnRange;
       columnRange.fPhysicalColumnId = i;
@@ -899,8 +902,10 @@ void ROOT::Experimental::Internal::RPagePersistentSink::InitImpl(RNTupleModel &m
    projectedFields.GetFieldZero().SetOnDiskId(0);
 
    RNTupleModelChangeset initialChangeset{model};
+   initialChangeset.fAddedFields.reserve(fieldZero.GetSubFields().size());
    for (auto f : fieldZero.GetSubFields())
       initialChangeset.fAddedFields.emplace_back(f);
+   initialChangeset.fAddedProjectedFields.reserve(projectedFields.GetFieldZero().GetSubFields().size());
    for (auto f : projectedFields.GetFieldZero().GetSubFields())
       initialChangeset.fAddedProjectedFields.emplace_back(f);
    UpdateSchema(initialChangeset, 0U);
@@ -943,6 +948,9 @@ void ROOT::Experimental::Internal::RPagePersistentSink::InitFromDescriptor(const
       }
       fDescriptorBuilder.AddCluster(clusterBuilder.MoveDescriptor().Unwrap());
       fPrevClusterNEntries += nEntries;
+      // Even though the number of entries are set when we call AddClusterGroup, we want them to be correct already
+      // at the end of this function.
+      fDescriptorBuilder.AddNEntries(nEntries);
 
       clusterId = descriptor.FindNextClusterId(clusterId);
    }
@@ -1133,6 +1141,7 @@ void ROOT::Experimental::Internal::RPagePersistentSink::CommitClusterGroup()
 
    const auto nClusters = descriptor.GetNActiveClusters();
    std::vector<DescriptorId_t> physClusterIDs;
+   physClusterIDs.reserve(nClusters);
    for (auto i = fNextClusterInGroup; i < nClusters; ++i) {
       physClusterIDs.emplace_back(fSerializationContext.MapClusterId(i));
    }
@@ -1156,6 +1165,7 @@ void ROOT::Experimental::Internal::RPagePersistentSink::CommitClusterGroup()
          .NClusters(nClusters - fNextClusterInGroup);
    }
    std::vector<DescriptorId_t> clusterIds;
+   clusterIds.reserve(nClusters);
    for (auto i = fNextClusterInGroup; i < nClusters; ++i) {
       clusterIds.emplace_back(i);
    }