why did old (like 90s/2000s) GPS take up to 15 minutes to get a lock, in giant bulky units, but my running watch gets it in 15 seconds?
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One reason (there’s a few more) is improvements in computing and signal processing algorithms. Old GPS receivers only had a few “channels”, let’s say 12. The GPS satellites transmit a signal that looks random, unless you know it. But even if you do, it took many minutes to just find the signal (this process is called acquisition) from one satellite, and you need to track four for about 30 seconds to learn about where it is (50 bits per second, pretty slow). That’s why it took forever back then. Today, you often have a network connection of some sort giving you the information about where the satellites are in a fraction of a second. And time and rough position. Even if it doesn’t, frequency domain acquisition algorithms searches through the full search space in seconds instead of many minutes.
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