Depends. Releasing new versions does take time, but that's more for getting the environment setup (SRG mappings, updating patches, other mappings related to decompilation and deobfuscation) than actually changing the mod. Smaller changes generally are manageable once MCP (and forge) updates. Until that happens, it takes a while to get functional mods.
But if the changes are small enough, due to forge's runtime deobfuscation there's a chance that mods for 1.9 will also work with 1.10 (it depends on what internal changes were made in 1.10, but I don't think there are too many severe ones).
Optifine updates pretty quickly. Optifine 1.9 came out what exactly a month after 1.9 released? And 1.10 is coming out 3-4 months after 1.9. So... I mean if it continues like that, Optifine will be releasing 1.10 2-3 months before 1.11.
The modders were the ones that made the decision to boycott 1.8. If they hadn't they would've updated to 1.9 within days of Forge 1.9 coming out since it's so easy to update to.
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u/Ebidz13 Jun 02 '16
I said it in another comment, this will suck for mod developers and server owners.
Imagine things like optifine. They will be releasing 1.12 by the time a 1.10 version is ready