Are there any "slow" sidechains? What are the tradeoffs?
Are there any &quot;slow&quot; sidechains?

Softchains fit this definition. Softchains are a type of sidechain that are verified by all mainchain users in a slow but extremely resource-efficient way, by only validating the blocks for which forks occur. It's slow, because you can't assume these forks will occur right away. People who want to actively use the softchain perform full validation, which means consensus will be fast for them.

What are the tradeoffs?

These softchains are not fully decoupled from the mainchain. Creating a new softchain requires a soft fork, and softchain consensus could negatively affect mainchain consensus under some scenarios (software bugs, one-year reorgs). Furthermore, mainchain validation cost goes up by roughly 100MB per softchain per year (which could be compensated for with a mainchain block size decrease).
The above is just a high-level overview.
For a relatively easy-to-follow explanation, I recommend listening to this podcast:
https://youtu.be/4c7cwTts3fo?t=1m45s
And the full details are in this write-up:
https://gist.github.com/RubenSomsen/7ecf7f13dc2496aa7eed8815a02f13d1