We recently switched our applications from using mongbd schema to the mongodb+srv and noticed significant performance degradation.

Simple performance test shows the following:
8 threads, 400 iterations
mongodb+srv :

44.8375ms connect time
    44.625ms get client time
    0.045ms get database time
    0.1ms get collection time
29.5575ms request execution time

mongodb:

3.7175ms connect time
    3.5325ms get client time
    0.01ms get database time
    0.16ms get collection time
23.8675ms request execution time

We are creating new instance of MongoClient for each request and the majority of time is spend on the client creation.

VS profiler shows that almost all time was spend on DnsClient.LookupClient constructor