1.15.2, 1.16.3, 20w49a, 21w03a, 1.16.5
When mined with a Silk Touch pickaxe, an Infested stone block will drop the regular version of that stone block, instead of dropping nothing.
I would expect, then, that mining an infested stone block with a silk touch tool would take as long to break as it would for that tool to break the regular version of that stone block.
It instead still breaks instantly, dropping the regular stone block, which can be confusing. This might be intended functionality, but I did not see any resolved bugs to this effect nor any info indicating this on the wiki.
1. Enchant a pickaxe with silk touch. Do not use sufficient efficiency/haste that regular stone blocks would be mined instantly.
2. Use it on stone blocks. Note that they do not break instantly, instead taking several ticks.
3. Use it on infested stone blocks, and compare. Note that they break instantly regardless.
Unknowns / Potential Limitations:
I have not tested this with infested versions of cobblestone or any stone bricks. The specific conditions under which I found this occurrence were with using a max-enchanted, named, diamond pickaxe in Peaceful Survival, and then replicated with a diamond pickaxe enchanted with only silk touch in a creative world switched over to Peaceful Survival.