While FTLDNS will pick its fastest responding server from the set of configured upstream servers on its own, it indeed doesn't supply corresponding statistics.
Lack of this data does not allow the user to make an informed decision on what servers he actually wants to keep.
However, response time observations may vary over time, so it's probably best to base your upstream DNS server on other factors like privacy, encryption or additional filtering, according to your personal preferences.
Then put a few servers that fulfill your criteria on your upstream list and leave response time optimisation to Pi-hole.
As you seem interested in the statistics themselves:
Neither pihole.log nor the current long term database hold response times, unfortunately.
Response time seems to be calculated for recent queries as shown by the Dashboard's Query Log only - they are not shown for queries in Long term data | Query Log.
A developer might be able to add additional insights.
You could also consider making this into a feature request.
You can preserve local name resolution by using Conditional Forwarding. You'll find that option on the bottom of Settings | DNS