The current logic for session restore relies on system memory pressure signals to stop loading tabs:

"""
void TabLoader::OnMemoryPressure(
    base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
  if (ShouldStopLoadingTabs())
    StopLoadingTabs();
}
"""

This signal has different semantics on different OSes. On macOS, it fires far too late.
https://bugs.chromium.org/p/chromium/issues/detail?id=713463