I've been spending a lot of time on one map, since I couldn't figure out what to do, and some Headcrabs that weren't anywhere about 2 seconds ago suddenly appeared and attacked me. So, does the game spawn in enemies to punish for taking too long on a map? I'm playing the Source version of HL1.
The original Half-Life does not punish you in any way for lingering around too long in any one area. Feel free to explore the maps as much as you want without fear of consequence for doing so. (though you may stumble upon a few things that you may have otherwise missed, such as enemies or invisible triggers)
The enemies that spawn in the map are placed in fixed positions (not dynamic), however there are a few instances where they will spawn an infinite number of times. There are also a few locations in the game where there is an invisible trigger which will spawn enemies the first time you pass through it. None of these are based on how long you stick around.
One such instance where enemies will continually spawn is on the surface when you are fighting the marines. They will come by helicopter and descend by rope. If you do not destroy the helicopter or move on past it, the helicopter will infinitely drop marines off as you kill them.
A trigger, if you're wondering, is an invisible box that exists in the game. If the player passes through this, it will cause a specific event to happen, such as an enemy spawning behind them, or a cinematic type experience such as an explosion.
There is one major exception to this, however; the game makes this situation abundantly clear and urges you not to dilly dally around, so it won't catch you by surprise.
When the portal to Xen opens for the first time, you must pass through it before it closes or the game will end and you will need to reload from a save. You will know when this is happening as there is a sequence of events leading up to this, and a scientist will yell at you, telling you to get in quickly.