The chunk-loading was addressed in MC-149178 and marked as "fixed" but I beg to differ. The mod comments in that report recommended creating a new report for 1.14.1, so here we go.

Lag is caused by loading chunks, and remains until the server is restarted. This generally takes several thousand blocks of travelling to replicate with a single player online, but can quickly bring a multiplayer server to its knees after just a few hours of standard gameplay.

In all of the following test cases, I am connecting to a remote vanilla server with a vanilla client. The server is hosted on hardware that is entirely separate from the client that is connecting to it.

Case #1

Buffet cave world with mob spawning enabled and 512MB of memory

Video: https://youtu.be/Zosu1BPpl6w

Highlights:
10:15 - See the lag in action (see case_1_during_lag.debug.txt)
12:15 - And how it disappears after rebooting the server (see case_1_after_reboot.debug.txt)

Case #2

Default world with mob spawning disabled and 1GB of memory

Video: https://youtu.be/AaGVQvC1vW4

Highlights:
20:50 - After travelling 16k blocks, the lag does not seem to have appeared (see case_2_before_lag.debug.txt)
23:30 - After travelling 20k blocks, the lag has appeared (see case_2_during_lag.debug.txt and case_2_during_lag.log.txt)
30:50 - The lag persists despite several different methods of attempting to alleviate it
36:00 - After rebooting the server, the lag no longer exists (see case_2_after_reboot.debug.txt and case_2_after_reboot.log.txt)