ROADMAP: http2 tests are merged, mention http2 perf

docs/ROADMAP.md

@@ -8,15 +8,11 @@ possible participation.
 HTTP/2
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-- test suite
-
-   Base this on existing nghttp2 server to start with to make functional
-   tests. Later on we can adopt that code or work with nghttp2 to provide ways
-   to have the http2 server respond with broken responses to make sure we deal
-   with that nicely as well.
-
-   To decide: if we need to bundle parts of the nghttp2 stuff that probably
-   won't be shipped by many distros.
+Improve performance. Measurements and tests have shown that in several cases
+doing transfers over HTTP/2 can be notably slower than the same transfer done
+over HTTP/1. Some of that difference can be attributed the inefficient window
+size handling currently in use but there are probably more to be learned and
+worked on to optimize this.
 
 HTTP cookies
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