Fewer bookmarks in each folder would work, as there is no way of setting a delay or a pause per bookmark.

Firefox has a setting that might help.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste max- and pause while the list is filtered

(3) Maybe: Double-click the network.http.max-persistent-connections-per-server preference and try 4 instead of the default value of 6?

The source code file has some notes on each of the different network.http.max-... settings which may or may not be helpful:

https://dxr.mozilla.org/mozilla-release/source/modules/libpref/init/all.js#1608

Decreasing network.http.max-persistent-connections-per-server mainly solves the problem. Thanks!