It looks like that laptop uses an NVIDIA chipset. Unfortunately, NVIDIA SATA controllers do not fully support AHCI and is limiting the performance of the SSD.
If you plug your Intel 320 into a system with an Intel/AMD controller with AHCI, you should see much better performance.... the sequential writes will be higher and the 4K numbers will be up.
I have the same issue besides applications freezing. What's the most compatible Intel/AMD PCI controller for this type of SSD?
Intel does not produce PCI storage controllers.
I would also advise you not to use a PCI controller because:
1) Most are ancient with limited driver support
2) PCI is limited to 133MB/s for the entire bus. All devices on the bus share that avaliable bandwidth so your SSD will be bottlenecked.
Thanks for the advice. I'll be looking at getting a Hybrid HDD instead and save the SSD for the nest PC. Any recommendation on an excellent Hybrid compatible with XPS720?