Instead of adhering to size constraints, the new versions are optimized for additional content released after 1.13, For one example, block type used to be a 8-bit char array but was changed to an arbitrarily sized map structure. This generally makes the game take more memory and more time to allocate it on the server side.
This doesn't necessarily mean that the lag on the server is caused exclusively by player count, chunk loading, random redstone stuff, or a worldedit job (it took about 100 lag machines to break a cheap test server). All of it together will cause lag,
Lag could be solved by changing the game to change how players play the game, but then there's a risk of gaining/losing players every time a rule is changed.
The server hasn't been laggy since release. Lag has come and gone with bugfixes and players finding new ways to break stuff.