Manually placing lava underwater using a bucket will cause a lag spike. As you can see in the attached images, every lag spike is a lava block placed and transformed in obsidian underwater.

Versions	Thumbnails	Versions	Thumbnails
1.12.2		1.17
1.13.2		1.18
1.14.4
1.15.2
1.16

I have recorded a video showing the performance issue while executing a metric capture. Both of them can be found in the attachments.

*1.18 UPDATE *
As of 1.18 update the lag spikes have disappeared. Manually placing lava underwater using a bucket doesn't cause them anymore.