Why reactive programming is so important?

How long it takes to do something for a computer? Answer

• execute typical instruction 1/1,000,000,000 sec = 1 nanosec
• fetch from L1 cache memory 0.5 nanosec
• branch misprediction 5 nanosec
• fetch from L2 cache memory 7 nanosec
• Mutex lock/unlock 25 nanosec
• fetch from main memory 100 nanosec
• send 2K bytes over 1Gbps network 20,000 nanosec
• read 1MB sequentially from memory 250,000 nanosec
• fetch from new disk location (seek) 8,000,000 nanosec
• read 1MB sequentially from disk 20,000,000 nanosec
• send packet US to Europe and back 150 milliseconds = 150,000,000 nanosec

Let’s translate them to some human kind of human time source:

1 CPU cycle	0.3 ns	1 s
Level 1 cache access	0.9 ns	3 s
Level 2 cache access	2.8 ns	9 s
Level 3 cache access	12.9 ns	43 s
Main memory access	120 ns	6 min
Solid-state disk I/O	50-150 μs	2-6 days
Rotational disk I/O	1-10 ms	1-12 months
Internet: SF to NYC	40 ms	4 years
Internet: SF to UK	81 ms	8 years
Internet: SF to Australia	183 ms	19 years
OS virtualization reboot	4 s	423 years
SCSI command time-out	30 s	3000 years
Hardware virtualization reboot	40 s	4000 years
Physical system reboot	5 m	32 millenia

So as soon as you are doing any IO you CPU can do lots of other stuff around.

No further reasoning….