Sorry to bring up an older thread, but much of the information here is highly inaccurate and is mostly due to people buying cheap flash drives and SD cards which are very hit-and-miss. I've tested in my Raspberry Pi 4b 4gb a Samsung EVO Select 64gb Micro SD, Sandisk Ultra 16gb Micro SD, and Kingston Canvas Go Plus 64gb Micro SD and they don't come anywhere near the performance of my Sandisk Extreme 3.0 16gb USB flash drive.

Here is the Samsung EVO 64gb Benchmarks: https://pibenchmarks.com/benchmark/51099
And here is the Sandisk Extreme 3.0 (for some reason it was identified improperly by the benchmark program. This doesn't change the performance numbers): https://pibenchmarks.com/benchmark/51100

Here is a link to the actual drive. https://www.amazon.com/SanDisk-Extreme-Flash-Drive-SDCZ80-016G-GAM46/dp/B00KT7DOR0

Booting from the Samsung Evo Select takes about 10 seconds longer than the Sandisk Extreme 3.0 usb stick.

That benchmark website pibenchmarks.com has tons of information on what drives are fast and what isn't. My USB 3.0 Samsung FIT drive is absolute hot garbage when running on a Pi 4. The fastest MicroSD cards do about 1700 points on the benchmark, There are USB sticks that score as high as 5,000 points. Then there are M.2 drives using USB 3 adapters which score over 10,000 points. Where USB Flash sticks and actual SSD/m.2 drives really excel is in the 4k random read/write tests which are the most important thing when it comes to performance.