Sometimes lawyers do preliminary motions like to suppress unconstitutional search warrants or change of venue and stuff. If it’s complex, it can take a while, and defense cannot request speedy trial if they’re filing things, but you also don’t necessarily want to forgo filing useful things.

Also, if they violate the constitutional right to a speedy trial, you can file a habeas corpus or something and, even if you win, there’s still no consequence except them shrugging and saying oops.