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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Something wasn’t right with the hardware you used in 90s/2000s and/or the hardware was operated too infrequently. GPS may need 15 minutes to get a lock only if it failed to receive a GPS almanac (a file describing long-term orbital parameters of the satellites) during the last 3-6 months. Normally a GPS receiver tries to save it in the background every time the receiver is on. If you operate a GPS receiver for 15-30 minutes continuously just once in 3-6 months it should update almanac and never take 15 minutes to get a lock. Even if it fails to receive an update, once updated it’s good for 3-6 months so a 15 minute long lock should not happen more than 2-4 times a year.
Besides the almanac a GPS receiver needs to get a GPS ephemeris (a smaller file describing short-term high precision deviations of satellite orbits). It is valid for 2-4 hours. It takes 30 seconds to receive it. A standalone GPS receiver that was off for 2-4 hours needs 30-60 seconds to get ephemeris before getting a lock.
To avoid the delay pretty much all smartphones download GPS almanac and ephemeris over wi-fi or cellular network either on demand or periodically. The file is only a few kilobytes so it takes less than a second to download it from an Internet server. That’s what your watch is doing to avoid 30-60 seconds delay to get ephemeris. It most likely downloads almanac and ephemeris via your phone. Unpair your watch, put it where it can’t get GPS signal, and you will see 45-95 seconds delay to get the first lock after 4 hours.
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