My hydrus collection is 1.9TB with nearly 1m files. Can't say I've ever had any kind of slowdown. A search with 24.8k results takes about 5 seconds, and that's with the media stored on a HDD (though the database is on a SATA3 SSD). And run by an 8 year old AMD Ryzen 7 2700X. Plenty fast enough for me at least.

ive imported around 2000 images, and it didn't take much, also take into account i only have 8gb of ram, so it did pretty well and pretty fast. first it crashed a few times but because i had a 300mb gif and it tried to read it and crashed. i have around 5000 images more but ill import them in batches because those are relatively organized in folders so it'll be easier to add tags as i go than dump everything and figure out what do to with them later. so far i dont notice any lag with my collection, during import it may take a bit more if your collection is larger tho but if you also have more ram... it also has Redis as an option for caching, i havent used it tho