I wanted to update my JDK, so I went to AdoptOpenJDK to download one. I saw that they have moved to Adoptium, and so I went over there. There I saw that there were no options to choose the JVM, and when I downloaded the installer, it was for Hotspot. I googled it, and I found IBM Semeru. Are these JDKs/JVMs different. Is there one which is significantly faster than the other? Does it even matter?
There's some discussion of the difference between the two from IBM over in https://github.com/docker-library/official-images/pull/10666#issuecomment-894204729:
Eclipse Temurin delivers OpenJDK + Hotspot images, and IBM Semeru Runtimes going forward will deliver both Open and Certified (JCKed) versions of OpenJDK + Eclipse OpenJ9 images.
It is a confusing story and transition, I will try to add a bit of background. AdoptOpenJDK up until now was producing OpenJDK binaries with both Hotspot and OpenJ9 VM's. With Adopt's move to Eclipse, legal restrictions prevent the new Eclipse Adoptium group from producing/releasing OpenJ9 based binaries. As a result, IBM will be producing OpenJ9 based binaries in 2 flavours, Open and Certified, both under the family name IBM Semeru Runtimes. Essentially the same binaries, released under different licenses.
So I suppose what you'll want to be looking for is comparisons between Hotspot and OpenJ9 (and I imagine any speed differences between those would be somewhat workload dependent, but there are quite a few articles online that have done actual benchmarks, such as https://www.ojalgo.org/2019/02/quick-test-to-compare-hotspot-and-openj9/).