The database of one of my Wordpress blogs can grow 20 MB in one week because of spam comments. They are all caught by Akismet, but I wonder what the influence on performance is (I have a Linode 512 LEMP)?
Basically I think it should not affect performance for visitors of my site whether the comment table (actually the commentmeta table) is 1 MB or 20 MB
Letting it accumulate does affect time to empty the spam comments, to the point of time-outs ( I let scripts run up to 60s). It does affect the size of the daily and weekly backups (especially when I had them emailed to me).

When does size begins to affect performance? Wordpress uses MyISAM.

That pretty much depends on:

- how well it is indexed
- whether row scans are involved
- hot data fits in memory
- no text type columns so any temp tables involved (eg. for sorting) do not spill to disc
- tps

Also, when possible, do not use MyISAM. Tuning for InnoDB is significantly easier, and it tends to handle writes better. Wordpress works absolutely fine with InnoDB.

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