Imagine user in San Francisco connects to New York via IPsec tunnel (Internet):
a) If I go via a MPLS network instead, is the latency about the same than the IPSec?
b) How about average performance? How many % performance decrease in applications should I consider when compared to MPLS given the encryption demanded by the IPsec tunnel?
This is a difficult question to answer precisely. What can be said is that there are no preformance guarantees on the Internet whereas with MPLS you will have some sort of SLA's with your service provider.
Other thing to bear in mind when comparing MPLS to Internet is availability.
As for performance, there will always be an additional overhead when using IPSEC but it can be somewhat alleviated by having a dedicated hardware module for the VPN encryption/decryption.