Calculate with exponents. A lot of back-of-the-envelope calculations are done with just coefficients and exponents, e.g. $c * 10^e$. Your goal is to get within an order of magnitude right that’s just $e$. $c$ matters a lot less. Only worrying about single-digit coefficients and exponents makes it much easier on a napkin (not to speak of all the zeros you avoid writing).

Latency Comparison Numbers
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(From 2017 in https://colin-scott.github.io/personal_website/research/interactive_latency.html)
The numbers of 2017 are easy to remember because of the exponents they use, look at "Read 1MB from X"

L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1KB with Zippy                  2,000   ns
Read 1 MB sequentially from memory       6,000   ns        6 us .006 ms  10^-2 ms
SSD Random read                         16,000   ns       16 us .016 ms
Read 1 MB sequentially from SSD*       100,000   ns      100 us   .1 ms  10^-1 ms
HDD Random read                      3,000,000   ns    3,000 us    3 ms
Read 1 MB sequentially from HDD      1,000,000   ns    1,000 us    1 ms  10^0  ms
Send 1 KB bytes over 1 Gbps network     10,000   ns       10 us  .01 ms
Read 1 MB sequentially from 1 Gbps  10,000,000   ns   10,000 us   10 ms  10^+1 ms
Read 1 MB sequentially from 10 Gbps  1,000,000   ns    1,000 us    1 ms  10^0 ms
Round trip within same datacenter      500,000   ns      500 us   .5 ms
Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms

Notes
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1 ns = 10^-9 seconds
1 us = 10^-6 seconds = 1,000 ns
1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns

Cost Numbers
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Approximate numbers that should be consistent between Cloud providers.

What    Amount  $/Month CPU 1$10
Memory    1 GB       $1 SSD 1 GB$0.1
HDD       1 GB    $0.01 S3, GCS 1 GB$0.01