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It pretty much has to do with what the game has to load in order to keep running "smoothly." Since there's more to display at a single time, in your case, the trees, it takes a little longer to keep it all running. Basically, it's lag. I had the same problem in my town when I had trees in practically every possible spot. The game also "lags" when there's multiple villagers within one spot. And it slows even more during an event. Say, with Tortimer, four villagers, and you at town hall plaza. Normally, when you navigate through your town, you'll notice as you travel southward, some trees and other structures disappear as others come into display before they become "hidden" by the rotund shape of the town. It's a big game, so it's loaded in sections.
Now, during WiFi, lag occurs for many more different reasons, some inculuding the aforementioned in the same case, not with the lag during dropping items. Sometimes, while over WiFi, it takes longer to drop an item or dig a hole or chop a tree or enter a building or pick up an item and so on because the change to the town has to occur to every visitor as well. So depending on the quality of the WiFi connection, it may take a while for that change to circulate through each system connected to the same town.
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