I made a mistake when i put, stops XP gains entirely.
I meant as in a ability/artifact that would decrease XP gains by -15% for 15 mins.
And i hardly ever put rank points into engineers now, they go to deck, its mainly cos of the NPC ratio.

How to slow rank:

Contribute to the base. Kill two birds with one stone, as it were.

how about any artifact that stops all EXP gains for 15 minutes but when the effect ends, 85% of the EXP that would have been earned will come flooding back and given to you, I'll like this while I'm NPCing and about to rank up.