Currently, I have 160K images in my catalogue and FWIW access and navigation are easy and quick. Though my images are all on an external DAS, configured in a five disk RAID array with dual disk redundancy, I have experimented with keeping the catalogue on both the external DAS and the native internal SSD of the iMac. I tend to favour keeping the catalogue on the DAS too (connected via Thunderbolt 2) as I will periodically switch to using my laptop for access to the catalogue. For me, Lightroom's organization capability is just so good I never give it a second thought. My bigger issue is probably just improving my file structures as time goes by. I've commented on this elsewhere in the forum but I think
@Steve 's video tutorial of the Library module is highly worthwhile. Unless you know the programme inside out, I believe its value is enormous.