External GPU
With an eGPU, you get:
- TB3 bottleneck, reducing performance (scaling with framerate and resolution - this is because of TB-PCIe conversion, not B/W) *
- CPU bottleneck, reducing performance (scaling with framerate) - only a few ultrabooks have anything approaching good CPU perf.
- Optimus/Enduro tech, means the iGPU still runs the display - meaning, amongst other issues (like colour control), you lose performance scaling with framerate.
- Higher cost - TB3 is usually reserved for premium flagship laptops,* plus you're paying for the enclosure (>15w PD is often pricey) and the card.
in cost of an egpu enclosure + ultrabook + gpu + psu you can get
- A laptop that performs significantly better, with way less bagagge
- A desktop build that performs significantly better
- An ultrabook + a desktop that performs slightly better
If you're happy with those things, or if they're outweighed by your need to have something ultraportable and with a great battery, then an eGPU may be right for you. Just remember that eGPUs are not for value-for-money, nor are they for the high-end (the performance losses become severe once you get past the xx60-ish performance tier). You will overpay and underperform compared to a gaming laptop - which is already overpaying and underperforming compared to a desktop - it's just a matter of by how much.
*(Ice Lake having an on-die TB3 controller appears to be improving things in these regards, however. Watch this space.)
