Site failing due to long _transient_rewrite_rules

My site, magmapoetry.com, has two problems:

1. it’s very slow (15 secs to load a page and my hosters say they see very high CPU load), and

2. if I let the server log errors, the log file fills all my 2GB of disk space rapidly with complains from mySQL that WP is trying to write data that is too large into the field called ‘_transient_rewrite_rules’ in the wp_options table. It’s trying to write several MB of rewrite rules. [“Got a packet bigger than ‘max_allowed_packet’..”]

Does anyone know how I can turn off whatever is writing that field and what the consequences will be? Or is there something else I should be doing? In principle I could raise mySQL’s limit on field sizes but I’d still take the speed performance hit from whatever writes it.

Thanks in advance for any help,

Tim