This 10 years notebook have geforce 305m 512MB but youtube seen not smooth, something hang when loading
so I wanna upgrade with egpu, dont have EC slot only mini pcie..
anyone can confirm it can support and no need external monitor.

Such an old laptop, is it really worth it for an adapter, gpu, power supply...

Try the h264ify extension to force youtube to use h264

or it seems like this laptop, some models have a free minipcie slot
could try looking for an old broadcom crystal hd decoder card
no idea if it will work well for modern os/modern day use
and even then it doesnt support vp9 so you still need the extension to force h264