What's the power loss to a squeaky chain?
This seems to answer your question:
https://cyclingtips.com/2020/05/how-many-watts-does-a-dirty-chain-steal/
It suggests a dry chain loses 7W out of 244W, which is 2.9%.
Your % speed loss will depend on your speed and aerodynamics - if you are producing, say, 80W, and cycling at 10mph on a flat road, then you could expect to lose almost that much time (so maybe another 100 seconds over an hour). At higher speeds, say 25mph on a flat road, then almost all of your losses are aerodynamic, so it comes increasingly true that your gains (or losses) are proportional to the square root of power losses, so you could expect to lose perhaps 1.5% of your time, so something like 54 seconds.
Of course stopping to lube isn't a bad thing as it helps avoid that little bit of premature wear to components meaning your chain lasts a few more miles, but  if it was raining or whatever, it is probably better to just keep going, as it would just wash off anyway.