Hi kuddel,For Orbiter 2016 just click the big green "download" button found here
which is R4.25 by the way.
Further down that page under "Downloads for Orbiter 2016" there is (at least) one more current version (R4.26 [beta] from October 9th 2021)
and some older releases as well.
Low framerates might occur when you've selected a "slow" (Software based - maybe) 3D device under the Video Tab
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