I think 8 was a release purely focused on performance, with some big improvements. Comparing 3.4 is kinda unfair.. You were fast with the tradeoff of half your data missing half the time
Consistent reads also mean additional checks. I think the WT cache is also very double edged. Completely cripples the database when resource constrained and not sized correctly, while giving a huge boost when the environment is "right sized". Honestly, give 8.0 and tell me how it compares. Haven't touched ancient Mongo versions in a long time, so I have little intuition on how it compares.