speedup encoder on q5-9 / 1MB+ files #317

Job	Run time
Build and test cmake:gcc9	51s
Build and test cmake:clang12	52s
Build and test cmake:clang12:asan	1m 19s
Build and test cmake:clang12:tsan	1m 22s
Build and test cmake:clang12:ubsan	1m 17s
Build and test cmake:qemu-arm-neon-gcc	2m 3s
Build and test cmake-osx:clang	1m 8s
Build and test cmake-osx:gcc	51s
Build and test cmake-win64:msvc2019-rel	1m 7s
Build and test cmake-win64:msvc2019-dbg	1m 39s
Build and test fuzz:clang12	40s
Build and test python39:clang12	2m 34s
Build and test python39-win	49s
Build and test maven	57s
Build and test bazel:root	48s
Build and test bazel:go	1m 1s
Build and test bazel:java	3m 10s
Build and test bazel:js	5m 34s
Build and test bazel:research	37s
Build and test bazel-osx:root	4m 12s
Build and test bazel-osx:go	4m 24s
Build and test bazel-osx:java	6m 6s
Build and test bazel-osx:js	8m 5s
Build and test bazel-osx:research	4m 34s
Build and test bazel-win:root	1m 4s
Build and test bazel-win:java	5m 52s
Build and test bazel-win:research	1m 28s
	1h 4m 24s

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