Can confirm that guardians try to swim in lava.
This is still an issue in 1.14.1.
And it's still an issue in 1.14.2 pre-release 1.
I have this problem too, in a world created in 1.14. The problem is still present in 1.14.1 pre-release.
[media]I have this problem, too. It occurs if I turn on chunk border rendering but also randomly happens in just normal spectator mode. I can eliminate the lag by going above ground or into a cave.
Squids don't swim in lava so why should Guardians?