100k nav cells causes extreme performance hit

I have 6 terrain measuring around 6km total with several nav volumes. If the nav cell count goes up to around 100k there is a significant delay in opening the map and PIE takes about 5min to run. (Press play, wait 5m, game starts)

If I increase the nav count to over 100k cells then the PIE time jumps up to 10m.

Removing the nav removes any issue.

Is there any sort of fix for this? Or am I expected to wait 1m for every 10k cells?

I havent had nav issues before with large areas and roughly 40k nav cells.