I have two different USB sticks, a 128 GB Samsung and a 64 GB SanDisk. I used Gnome Disks to benchmark both of them and the Samsung one was supposedly better, it had faster read and write speed but it seems the SanDisk one had faster access time, although not by much. So I installed Ubuntu 18.04 on the Samsung drive using the default Ubuntu installer. The drive was formatted in MBR mode, so Ubuntu installed GRUB in legacy mode. I had no swap partition nor a swapfile.
Well, I used for some weeks that USB to boot Ubuntu on my lap top, and I noticed some performance issues. Sometimes the system would hang for a few seconds while loading or closing a program or simply when I opened a mail in thunderbird. I know that performance is not as good as if I installed it directly on the Hard Disk but I've used Arch Linux from a different USB stick before and the performance wasn't that bad.
So just to see what would happen I formatted the SanDisk drive in GPT format with an EFI partition, copied all the files from the Samsung drive to the SanDisk one (using this Arch Wiki article as a guide, it preserves all permissions, etc) and installed GRUB in EFI mode. Technically I have the same Ubuntu installation, just on a different drive and bootloading in GRUB EFI mode instead of GRUB legacy MBR mode. But after using it for a while the performance is way better, so far it has not hanged in any way.
How I can diagnose the cause of the difference in performance? Does booting in EFI mode helps performance? Is the Samsung drive defective? Is something else?