There's only a significant cost to synchronous MTE.

MTE ships two modes. synchronous mode and asynchronous mode. SYNC is slower but gives you far better traces and throws an SEGV_MTESERR as soon as violations happen. ASYNC however is async so there's a bit of a delay between a violation and the "catch" that throws SEGV_MTEAERR.

Strictly speaking async is worse for security because there's a brief window of time where the process "gets away with it" but the main differentiator is that because things don't stop the moment the violation occurs, SEGV_MTEAERR traces tend to be some degree of "out of date" vs SEGV_MTESERR which capture the exact state of the world the moment the error occurs.

The main tradeoff here is that async MTE has basically negligible cost. Something like 1-5% in practice but in microbenchmarks you can see up to 50%. Vs synchronous MTE where the penalty is on average closer to like 10-15% but in microbenchmarks it can be like 5-6x slower.

So yeah the perf cost is there but it's really not a major issue.

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The main issue is that apps and services crash when an MTE segfault occurs. So this means that to the uninformed end user apps appear spuriously unstable with no meaningful context. And it's not just apps. On graphene I see MTE segfaults semi regularly from various system daemons (mainly related to GPS/nav) and occasionally in Google Play Services itself.

From time to time I get them in Youtube and I constantly get them in the Twitch app. It's very annoying and the apps just crash when you happen to do some particular action leaving you walking on your toes to avoid accidentally tapping whatever magic pattern happens to invoke a MTE SIGSEGV until the next update. Doubly so since most apps provide no meaningful interface for uploading log traces to report these issues.

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If Google wanted to roll this out without the spurious crashes they'd need to deploy it and eat the minor perf hit but they'd need to register a global signal handler to capture these MTE SIGSEGVs and report them back to the services in question without crashing the app.

And most app devs don't care so they'd be taking a minor (or major) perf hit in exchange for logging errors that developers always ignore. Doubly so in the modern day of "just have AI fix/do XYZ and who cares about the consequences as long as it runs".

I'm not surprised they dropped it but I do honestly wish they'd forced the issue and just deployed it and forced apps to fix their shit.