don't compute InterUnitBonds when parent ones empty

CHANGELOG.md

@@ -14,6 +14,7 @@ Note that since we don't clearly distinguish between a public and private interf
 - Change line geometry default ``scaleFactor`` to 2 (3 is too big after fixing line rendering)
 - Trajectory animation performance improvements
     - Reuse ``Model.CoarseGrained`` for coordinate trajectories
+    - Avoid calculating ``InterUnitBonds`` when ``Structure.parent`` ones are empty
     - Reuse unit boundary if sphere has not changed too much
     - Don't show 'inter-bond' and 'element-cross' visuals in line representations of polymerAndLigand preset
 - Fix additional mononucleotides detected as polymer components

src/mol-model/structure/structure/structure.ts

@@ -1,5 +1,5 @@
 /**
- * Copyright (c) 2017-2021 mol* contributors, licensed under MIT, See LICENSE file for more info.
+ * Copyright (c) 2017-2022 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author David Sehnal <david.sehnal@gmail.com>
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
@@ -232,7 +232,12 @@ class Structure {
 
     get interUnitBonds() {
         if (this.state.interUnitBonds) return this.state.interUnitBonds;
-        this.state.interUnitBonds = computeInterUnitBonds(this, { ignoreWater: !this.dynamicBonds });
+        if (this.parent && this.state.dynamicBonds === this.parent.state.dynamicBonds && this.parent.state.interUnitBonds?.edgeCount === 0) {
+            // no need to compute InterUnitBonds if parent's ones are empty
+            this.state.interUnitBonds = new InterUnitBonds(new Map());
+        } else {
+            this.state.interUnitBonds = computeInterUnitBonds(this, { ignoreWater: !this.dynamicBonds });
+        }
         return this.state.interUnitBonds;
     }