IMO, depending on what you do, you might take a huge performance hit or you might notice no difference at all.
The 2017 13-inch would be expected to have somewhat faster single threaded performance, but dramatically slower multithreaded performance, compared to the 2016 15-inch. The 15 would also be expected to have superior video performance. Additionally, the base 15 has 16 GB of RAM, versus the base 13's 8 GB, which in certain usage situations can be night-and-day. Both would have similar disk performance. The 2016 15-inch should have a runtime advantage over the 2017 13-inch with most usage.