Unreal is probably CPU bound. The graphics card is probably much faster than the CPU and it’s falling behind with calculations, causing it to not serve the GPU new information fast enough.
You can use the stat unitgraph command to get information regarding which part of the engine is causing the slowdown
Result of stat unitgraph
The slow part of the calculations will be marked in orange or red.
If it’s game loop calculations then it’s usually the CPU that is bottlenecking the system.