Minecraft – How to execute a command at an random entity

What I would do is summon another ArmorStand under and increment all existing ones within certain range, maybe just those that have same X and Y coordinates. That way it would not interfere with other existing goalposts.

Alternatively, once you reach ground, lock all existing goalposts (by using another scoreboard objective). When you create new goalpost, you could again summon ArmorStands under and increment the score for those that are not locked yet.

It's server lag; the server is given a huge amount of work and it will take longer to finish the job, which causes most things to grind to a halt until it's finished.

The command parser will separately execute a command for each individual target obtained by a selector.

For example, when running the following while there are 1024 armor stands:

/execute @e[type=ArmorStand] ~ ~ ~ /testfor @e[type=ArmorStand]

There are 1024 /execute commands being processed, and then for each and every /execute command processed there are 1024 /testfor commands processed (totaling 1,048,576 commands processed). This is an extremely large amount of work that needs to be done, and it's only a single /execute.

The more nested executes you use, the worse off it's going to be. Not only do the commands have to be processed thousands of times, the selectors do as well (which comes with some heavy operations, such as sorting all targets by distance before picking from them).