Recently upgraded a 16.04 Ubuntu guest (Windows 10 host) to to 18.04 and saw a massive drop in performance. The lag in the desktop environment was unbearable. This was even with the guest additions properly installed. These settings finally fixed it:

• System
  
  • Motherboard
    • Chipset: ICH9
    • Extended Features: Enable I/O APIC, Hardware Clock in UTC Time.
  • Processor
    • Extended Features: Enable PAE/NX
  • Acceleration
    • Paravirtualization Interface: KVM.
    • Hardware Virtualization: Enable VT-x/AMD-V, Enable Nested Paging.
• Display
  • Disable 3D Acceleration
• Storage
  • Controller: SATA
    
    • Use Host I/O Cache enabled[1]
• Network
  • Adapter 1
    • Advanced – Adapter Type: virtio

[1] This can lead to wasted memory via double-caching since the guest and host have their own I/O cache, among other tradeoffs.